18th Dec 2015

“Do you think incest and polygamy will be legalized?” – Part 1

I came across a poll on the subject of the future of full marriage equality a while back:

I was watching a TV show recently and surprisingly, three characters (two men and a woman) got together in a sexual AND romantic relationship – as in, it wasn’t just a kink, they were serious about it, they even made their relationship official to their family/friends. And it got me thinking, do you guys think polygamy will ever be legalized? And incest? And should they be, and if not, why? Will our society ever see it as acceptable/normal like with gay relationships?

My response:

First of all, I’d like to point out that in some ways this debate is moot:polygamy in some form is already legal in many countries, sex with a close relative is legal in some, and marriage to your half-sibling is legal in Sweden. Any push for more legal recognition in other countries would just be a continuation of what already exists. 

Futurist, on 25 Apr 2014 – 12:31 PM, said:

I’m honestly unsure about this because there might not be enough demand to legalize either of these two things.

Actually, there’s demand for both, because there are people in either type of relationship (sometimes in both at once), and their relationships are extremely illegal in many places. In some statessimply cohabiting with multiple people of the opposite gender counts as a common-law marriage, and can get all of you sent to jail for up to 10 years.In most Southern states, as well as some states in Australia, a single sex act with a consenting adult you are related to, regardless of whether you two grew up together, can get both sent to jail for life. There is also a growing community of allies, many from the LGBT+ community, who support relationship and marriage freedom for all consenting adults, regardless of any qualifiers. 

My blog
My friend’s blog

There are also other people publicly speaking out against discrimination and legal abuse. Some are even trying to change culture through positive media representations

They’re real people, and under the current legal regime and set of taboos they suffer, sometimes brutally. Just because you don’t personally know of them doesn’t mean they aren’t there. In fact, you may know some and just don’t know you know them. Consanguineous couples especially, are deep, deep in the closet.

I’d also like to point out that according to surveys (and some testimony from people I know), as much as 10% (possibly more) of the population in the US has at least sexually experimented with a sibling (non-coercively, of course). Anyone who advocates for “incest” laws is advocating for throwing as much as a tenth of the entire population into jail.

Death, on 25 Apr 2014 – 8:31 PM, said:

If we could overcome the genetic problems then would incest be accepted?  Not advocating for it definitely, because incidence of birth defect is very high (somewhere above 50% I forget and we are talking serious die before you’re 5 years old birth defects and major retardation).

First of all, genetic testing and family planning does allow people to overcome “the genetic problems”. Sometimes there are no genetic problems. Your statistic is also incredibly high, and smacks of stereotypes and not science. Of all the reasonably well done studies I’ve seen, the most pessimistic estimate for genetic problems for 1st degree relatives is 30%. The more realistic estimate I’ve seen is around 11%, […] lower than for some middle aged women. (That’s for a single generation, of course.)

Those are population statistics anyway. Individual couples may be more or less likely to have problems, just like the general population. Also, there are people in the general population who are not closely related, but both possess terrible genetic diseases which they can pass on to their children, but we don’t forbid them from getting married or throw them in jail for having sex. Scientists who know anything about this subject think the eugenic argument is bunk. (I personally find eugenic reasoning to be anti-democratic. It also allows a return to eugenics based on things like race and class). For one, even for the best studies, everyone is aware that their samples are so small and so biased that they can’t really even conduct a statistically reliable study beyond 1st cousins, since in most places closer unions are illegal. 

“Major retardation” is not the most common defect anyway. The usual problems are related to the immune system or to basic organ function, like the heart. Most of these children are totally fine, and the ones that aren’t are still mentally normal. Who is anyone to determine that their lives have no value to themselves or society, and should be prevented at all costs, even at the cost of undermining human rights? Besides, not all consanguineous relationships are even heterosexual.

Alric, on 25 Apr 2014 – 7:23 PM, said:

They are stupid laws. You should be able to do whatever you want with other consenting adults. Marriage is just a contract as well, if you want to marry out of love in a straight or gay relationship, or marry for money, or to get citizenship or because you made a bet, all those should be consider valid reasons. The government shouldn’t be deciding who should or shouldn’t get married, they should only be recognizing all marriages people ask them to recognize.

Amen! I’d also like to point out that many people don’t feel disgust anymore towards the idea of homosexuality is because it’s seen frequently in public.Many people still do feel disgust at the idea, even when they support same-sex marriage, and many more felt disgust at the thought in the past. It’s a known mechanism, in which seeing another person do something you wouldn’t do makes you imagine yourself doing it. The revulsion felt at that mental imagery causes people to lash out at the people who inspired the thoughts. It’s the same social psychological mechanism, though perhaps for consanguineous sex the revulsion is stronger for some people.

“Naturalness” also isn’t a good rubric for these things. While most animals avoid sex with close relatives in nature, not all do. While most animals have sex with an animal of the opposite sex, not all do. Polygyny, polyandry, and polygynandry all occur in nature. (Polygyny is much more common than monogamy in nature. Should monogamous marriage be illegal?) Which is natural, the majority, or the minority, given that both occur in nature? Besides, neither homophobia, nor the “incest” taboo, occur in nature outside of human culture. Same-sex and consanguineous sex acts are more “natural” by many standards, than the taboos and laws against them. 

“Sexual morality is too important to the happiness and the well-being of us all to be determined by superstition, politics, economics, and religious taboos.”

ayubelwhishi, on 25 Apr 2014 – 06:29 AM, said:

Its still a a taboo for me because you can get better women than your sibling or you mother. I just see incest as a pathetic way to love. Like how does the wheel of your love life land on your sister? Don’t reply on this because I’m too tired to want to get in an argument over it.

On polygamy, i believe it should be legalized if the woman consent with it. But i do not know how a woman would like to love a man who love other woman. It seems like a way to cheat without making her mad.

A lot of assumptions there. What if your mother is the best woman, objectively? What if you meet them for the first time long after you’ve grown up? Not everyone, even the ones who get involved with family, are ugly, antisocial, or can’t get anyone else. Some have already been married before to other people. Besides, “better”? Are we cattle? Are we judging marriage purely by economic status and social acceptance? That’s a great way to produce loveless, alienating marriages. Wonderful for the children, I’m sure, to have one of your parents pining the whole time for someone else. 

First of all, “a woman” doesn’t have to agree to anything. In the example for the poll, the union is polyandrous – multiple men, one woman. Why aren’t you asking how “a man” would agree to it, unless you have a gendered conception of promiscuity and consent, where only men want sex with various people, and women have to be cajoled? And it is not “cheating”, by definition. Cheating is having sex with someone else without your partner(s)’s consent. Key to the idea and the term “cheating” is going behind someone’s back. Having sex with a second spouse, whom your other spouse knows and approves of, with the knowledge of your other spouse, is not “cheating”. 

People also tend to be less threatened when the other person is someone they know and are friends with. People have different levels of natural jealousy, and jealousy can be mitigated with proper conversation and conflict resolution. People get jealous when they feel their own emotional and sexual needs aren’t being met/won’t be met. Jealousy, for the exact same reasons, exists in monogamous relationships as well. Jealousy is just more obvious in polyamorous relationships. As a result, people who’ve been poly* for a long time are usually much better at dealing with jealousy and talking honestly about their feelings with their partners than many monogamous people.



The naturalistic fallacy needs to die

The “naturalistic fallacy” is commonly used against both biological science, homosexuality, and consanguinamory. It’s a fallacy for a reason.

If “natural” is “good”, what use does it serve as a moral rubric? When people say “homosexuality is unnatural”, they are saying “homosexuality is wrong”. But how does that help the discussion? “It’s wrong because it’s unnatural” is the same as saying “it’s wrong because it’s wrong”. That doesn’t tell us why it’s wrong: it’s again “a description” – a false one in this case, since […] there are 1,500 animal species that engage in homosexual behaviour. The assertion becomes a tautology. But just showing that a supposedly unnatural act occurs in nature does not make it moral either! The entire point is to get rid of linking so tightly “moral” and “natural”: whether something does or does not occur in nature doesn’t aid our deciding whether that act is moral.

After all, wearing glasses, building hospitals and using crutches don’t occur in nature – are these to be considered “wrong” based on that category? If they’re not, why use the category of “unnatural” or even “natural” at all when discussing morality?

Tauriq Moosa



“Genetic testing breathes new life into Israel’s Samaritans”

Samaritans celebrating Sukkot

All of these weird eugenic arguments people make about consanguinamory, about people having “too many kids,” are not only creepy, but they’re based on bad assumptions. When people are given easy access to good information on how to manage their families, they usually use it. The results can be amazing.

MT. GERIZIM, West Bank — When Ben Yehuda Altif got engaged to his first cousin Mazal, there was no problem winning the blessing of their families or the Samaritan high priest, who leads their ancient Israelite sect. Marriage between cousins is common in the religious community. But there was still an obstacle. Like many Samaritan couples today, the pair had to pass a premarital genetic screening to predict the likelihood of having healthy children. Without the green light from doctors, the marriage would be off. “Doctors said OK, and now we have a healthy, handsome boy,” said Altif, 33, reaching for his wife’s cellphone to show off pictures of their son.
Samaritans, who trace their roots back about 2,700 years, are best known for clinging to strict biblical traditions that have largely disappeared, including animal sacrifice, isolation of menstruating women and, until recently, a ban on marrying outsiders. But after facing near-extinction and being devastated by a high rate of birth defects because of inbreeding, the community is using modern science — including genetic testing, in vitro fertilization and abortion — to preserve their traditional way of life.
“It’s changing our blood,” said Aharon Ben-Av Chisda, 86, high priest of the 750-member Samaritan community, which is split about evenly between the West Bank village of Kiryat Luza near Nablus and the Israeli city of Holon, south of Tel Aviv. The white-bearded priest said genetic testing was breathing new life and optimism into the once-besieged community. He noted that he and his wife, who is a second cousin, had four children before genetic testing was available: Three are deaf and one can’t walk. Most other families at Mt. Gerizim tell similar stories of health problems and handicaps among the older generation, though lately such problems have begun to disappear.
Samaritans are one of the world’s oldest religious sects. Similar in practice, beliefs and ancestry to Jews, they follow the Hebrew Torah. But instead of Jerusalem, they revere a temple their ancestors built on this remote West Bank hillside.
Mentioned several times in the Bible, Samaritans are also considered one of the most inbred communities in the world, with 46% marrying first cousins and more than 80% marrying blood relatives, according to research by Israeli geneticist Batsheva Bonne-Tamir, who spent most of her career studying the community. The restrictions against marrying outsiders were less of a problem when Samaritans numbered more than a million in the 5th century. But because of persecution and forced conversion to Islam, their numbers had dwindled to just 146 by 1917. To crawl their way back, Samaritans began having large families of eight to 10 children, and the frequency of first-cousin marriages doubled, Bonne-Tamir found. As the population grew, so did the health problems and genetic defects, including rare blood diseases, Usher syndrome, deafness, muteness, blindness and physical handicaps.
“It was largely a 20th century phenomenon,” said Bonne-Tamir, now retired from Tel Aviv University. Over the last decade, the community also relaxed its restrictions on intermarriage, allowing in about 25 women, mostly Jewish Israelis and arranged matches with brides from Ukraine. Samaritan leaders are reluctant to discuss their gene pool shrinkage, but they estimate the rate of birth defects was once 10 times higher than the nationwide average. By the 1960s, the rate of miscarriage was 10% higher among Samaritan women, one study found. But since adopting genetic testing, Samaritans say the rate of birth defects among newborns today is normal, even though most people still marry inside the community, including to relatives.

“This is enabling us to build a better generation for the future,” said Ishak Al Samiri, a spokesman for the community at Mt. Gerizim.
Like his father, Al Samiri married a cousin. He has two healthy children, but he suffers from a blood disorder and his brother is crippled, both believed to be linked to genetic defects, he said.
Samaritans have long been the focus of genetic research, initially because of their ancient roots. In the 1960s, Israeli scientists began to study the Holon branch of the community, both to assist with genetic defects and to trace their historic lineage.

Samaritans claim that they are the descendants of northern Israelite tribes that were conquered by Assyrians. Subsequent genetic studies suggested that Samaritan men carry the so-called Cohen gene, linking them to ancient Israelites. For centuries, Samaritans lived in Nablus, but some moved to Jaffa and later to Holon. In 1988, the Nablus community relocated to a village near an Israeli settlement to escape attacks by Palestinians, who viewed them as Jews. Today Samaritans, who hold Israeli citizenship, pride themselves on staying neutral in the Israeli-Palestinian conflict.

This can happen if there’s a sudden, extreme bottleneck. Moderate levels of consanguineous marriage can have little to no consequence on a population’s health stats, as long as the population is large enough and there is still non-consanguineous marriage. If a bottleneck does happen, it can be many generations before all of the deleterious genes from the founder population are eliminated from the gene pool. One of the ways to get around this, as the Samaritans have shown, is to use genetic tests to inform marital and reproductive decisions. If people are worried about children born with disabilities – and I’m assuming they’re genuine here, and not just using this as an excuse to attack consanguinamorous people specifically – then a great way to address those concerns is with widespread, cheap access to genetic testing and family planning. As the Samaritans have shown, if you have that, consanguinamory isn’t much of a problem.



Unfamiliarity leads to disgust, and disgust leads to hate

Westermarck suggested that humans have an inclination to prevent other people from behaving in ways they would not themselves behave. On this view, left-handers were in the past forced to adopt the habits of right-handers because the right-handers found left-handers disturbing. In the same way, those who were known to have had sexual intercourse with close kin were discriminated against. People who had grown up with kin of the opposite sex were generally not attracted to those individuals and disapproved when they discovered others who were. […] Once in place, the desire for conformity, on the one hand, and the reluctance to inbreed, on the other, would have combined to generate social disapproval of inbreeding.

– Patrick Bateson, Inbreeding, Incest, and the Incest Taboo

Why do we condemn others for having sex with their relatives? What has this to do with our not being interested in having sex with our relatives? […] We condemn them because by arousing our aversion their behavior causes us pain.

– Arthur P. Wolf, Inbreeding, Incest, and the Incest Taboo



Help for Family and Friends of Incestuous Siblings

This is for the benefit of friends or family of romantically involved siblings, who may have recently discovered their secret. Though I’ve used “incest” in the title, I won’t continue to use the terms “incest” or “incestuous,” I will use “consanguinamory” and “consanguineous” (pronounced “con-sang-gwin-am-or-ee” and “con-sang-gwin-ee-us). “Incest” is too loaded a word for intelligent discussion, and I only ever use it for sexual abuse. If I say “consanguinamory”, assume I am talking about consensual sex. (I’m going to assume that the couple is opposite-sex, but most of this also applies for same-sex couples.) Remember: there’s a difference between love and abuse.
This might be long, but bear with me. All of your concerns are about to be addressed. If you truly love them, you will have the patience to read this.

First, stop and take a breath. I know that this must be a lot to take in. I seriously doubt that you’ve ever sat down to consider the possibility of this happening. I don’t expect you to be calm, but I do expect you to care enough about their well-being to seriously consider what I’m about to say.
Did you discover them accidentally? If so, talk to them individually – with an open mind – and make sure that there was no coercion. Ignore the taboo nature of what you just found out. If you have no evidence of coercion or manipulation, then do not try to project abuse where there is none, and do not force them to internalize your own sense of what’s “taboo.” Why would you ever want to burden them with so much unnecessary guilt and shame? Talk to them together, and get the story from them, calmly. See how they act together. Remember to treat them with respect, especially if they’re already adults; it’s what you would want for yourself.
Did they come out to you on their own? Then there’s even less chance that there was any coercion involved. In fact, coming out to you is one of the bravest and most trusting gifts they could ever give you. Not only is their love extremely taboo, but even if they are adults, in most places on Earth they could be thrown in jail, possibly for the rest of their lives. You could get them thrown in jail. Every person they tell is a potential threat who could ruin their lives forever, getting them locked up for years and permanently placed on the sex-offender registry. And yet, despite all that, they told you. They could have lied – it wouldn’t have been easy, but they could have – but they told you. However much you thought they trusted and loved you, they just proved that their true trust and love is greater.
If they say that it’s consensual, and there’s no evidence it isn’t – especially if they came forward on their own – how can you still assume that no person could consent to it? How can you possibly disrespect their intelligence and agency so much? Have you ever had any other reason to doubt that they are of sound mind and soul? Then why should this one thing counteract years of personal experience? Did they hurt anyone? Of course not. If you think there must be something wrong, it’s because that’s the story society has been spoon-feeding you.
Consider: if one of them was adopted – if they weren’t genetically related – would you still feel as uncomfortable as you do? Because if you wouldn’t, then there’s no good reason for your discomfort now; socially, whether adopted or not, their relationship would be the same. If they weren’t even raised together, then in no way are they family, though they are blood relatives. Ignore for a moment the particular, taboo nature of their relationship. Just consider them as individual people. If your daughter/sister/friend was dating a man like her brother, knowing everything you do about him, would you be displeased, or happy? If your son/brother/friend were dating a woman like his sister, knowing everything you do about her, would you be upset, or glad?
If you are their parent, unless you’ve done an awful job of raising them, my guess is that, before you found out, you were quite proud of them. Well, they’re the same people now, the same people who made you proud. Wouldn’t you want your daughter to date a man who made you as proud as your son? Wouldn’t you want your son to date a woman who made you as proud as your daughter? Aren’t they more to you, and to each other, than just their genes?
Society has taught you to feel a certain way about consanguinamory. It was handed to you, and you accepted it without much thought. You’ve probably never met anyone who was openly sexually involved with a close family member. This has allowed you to go around without seriously considering what such a relationship might look like, how it could work, and how you should feel about it. It has allowed you to absorb the limited perspective put out by the media, giving you a narrow, stereotyped view of what’s possible. You have been listening to only one side of the story your whole life.
Just because you don’t know that you’ve met such a couple before, doesn’t mean that you haven’t met one. In fact, as you follow your family tree further and further back in time, the probability that you will find at least one consanguineous couple approaches 100%. Self-reported surveys have found that as much as 10% of college students have had consensual sexual contact with a sibling (mostly childhood experimentation). (If we extrapolate this to the whole population, this equates to about 30 million people in the U.S.) The fact that a couple is related tells you exactly nothing about what their relationship is like, nor whether it is consenting or not, nor whether it is fulfilling or not. Each of those things is independent of their blood relationship.
The cultural stereotype of such relationships is that they are dysfunctional, self-destructive, and abusive; anyone who willingly participates must somehow be mentally ill. Besides this view being incredibly condescending, it also has no meaningful basis. What is considered “healthy” and “unhealthy” changes, and is very subjective. On what standard are we to decide what constitutes mental “illness?” Is it that they’re doing something they know society disapproves of? I don’t think any reasonable person thinks we should use the preconceptions of the majority to decide what constitutes mental illness. It must be, then, that the behavior is self-destructive, or causes them to destroy the lives of others.
Do you see anything indicating that those things are happening? Aside from their experience of bigotry, do they seem unusually disturbed? Are they lashing out at themselves, at each other, or at you? Are they unable to operate normally in a social environment? If not, then you have no reason to think they are any less mentally healthy than before. In fact, their love may have made them healthier, by bringing them fulfillment and peace.
As I’ve said, you’ve probably already met a consanguineous couple. They couldn’t have stood out as any more dysfunctional than the average couple, or you would have become suspicious that something was wrong. Unfortunately, prejudice keeps people in the closet, which perpetuates ignorance, which itself perpetuates prejudice. You have been given the rare opportunity to examine your own assumptions, and break your own cycle of prejudice. Most people have never gotten that chance.
The “pedophile” label has long been used to brand sexual minorities as deviants, as threats to society and to our children. Homosexuality has long been heavily attacked as pedophilic, and in the past when people had limited experience with open, healthy same-sex relationships, they believed the propaganda. Now that so many homosexual couples are out in the open, we realize that there is a clear difference between the consenting majority, and the predatory minority.
Even today, opponents of legal rights for homosexuals try to brand the gay rights agenda as pro-pedophilia. There is a homophobic Neo-Nazi “vigilante” group in Russia called “Occupy Pedophilia,” but it isn’t pedophiles they’re targeting: they target young gay men. They go around torturing them, sometimes to death, and use “fighting pedophilia” as their implicit justification.
It is the same for consanguinamory. The vast majority of cases that come to light are the most unhealthy. (In the previously quoted summary of studies, only 30% of respondents answered that their reaction to sexual contact with a sibling was “negative.” Of that 30%, 25% were non-consensual. The remaining 5% may be due to stigma and shame.) Those in healthy, fulfilling relationships never come forward, and we only see them in the news when they are caught and thrown in jail.
The consanguinamorous are lumped in with a predatory minority, and because of the closet, the public buys it. Just because these siblings love each other, it doesn’t mean that they want to have sex with any other relatives, and it doesn’t mean that they are pedophiles. Despite the propaganda, their relationship does not automatically mean they are abusive and emotionally damaged.
Besides, so what if every other consanguineous relationship in history has been abusive and emotionally damaging? We consider people as individuals, and don’t punish them based on the sins of others. Even in murder trials, attenuating circumstances are considered. If murderers get the benefit of the doubt, if murderers get to be treated as individuals, then why not these siblings? Even if every other relationship like theirs was damaging, that doesn’t automatically mean theirs is. If they are the only loving, consenting blood-related couple in the world, then that’s all the more reason to treat them with respect and dignity.
However, they are not the only siblings to have a consenting, loving relationship. It is not some newfangled idea. Societies’ attitudes towards various sexual relationships – especially familial – have changed all throughout history. They are in illustrious company, among some of the greatest people to ever live. These are just a handful of the examples known, and there are certainly many more lost to history.
Not only are they in glorious past company, but in beautiful present company as well. In the past, only royals and aristocrats could break society’s rules and marry whom they wished. Why should the right to love whom they wish to love be denied to the common man or woman? Romantic sibling relationships are much more common than most realize. Many of these relationships, when allowed to flourish, grow into something astoundingly beautiful.
You may wish that they would just find other people. There are plenty of non-blood-related fish in the sea. If they did that, it would certainly make things easier for you, wouldn’t it? You may even be able to convince yourself that it would somehow be easier for them, too. Well, why should they find other people?
Do you have someone you love? If so, why don’t you find someone else? It’s easy to see that it’s not so easy. If you knew a bisexual man who was dating another man, would you tell him that, because he has “more acceptable options,” that he must date a woman? The “homosexuality isn’t a choice” argument is strawmaning: it serves as a nice talking point, but that’s not ultimately why society now feels that homophobia is wrong. We’ve come to understand that love doesn’t always fit the conventions proscribed by society; that it is morally wrong to police people’s sex lives and love lives; that society is better off when we nurture people’s natural love. A bisexual person may be capable of loving someone of the opposite sex, but that doesn’t mean they will. No-one chooses who they fall in love with. It is no different for siblings in love.
Besides, have you stopped to consider the consequences of forcing them to break up? People think only of the consequences of letting siblings stay together, but not of destroying their relationship. Consider: how will breaking them up, causing them misery and pain, shaming them, and policing them make their relationship “healthy?” Even if you think it’s “unhealthy” now, their relationship is guaranteed to be much worse after that kind of trauma. They’ll remember what they had, they’ll remember the pain of its loss, they’ll remember the judgment, they’ll remember the shame, and they will probably know that they still love each other. What kind of family dinners do you expect with that kind of angst floating around? They may in fact choose to never see each other again, because it would be too painful.
What if they shun your judgment and shaming? Many consanguinamorous couples, when facing judgment and intervention by friends and family, break off all ties with them for the sake of preserving their own relationship with each other. If you really do care about them, and also want to be part of their lives, learn to at least tolerate their love. Better that you have a presence in their lives. Don’t force them to choose between family and friends, and the love of their lives.
Now, there is one legitimate concern regarding consanguinamory: won’t introducing sex and romance destabilize the family dynamic? What if it ultimately doesn’t work out? Won’t that make it difficult to go back to being just family for them? The short answer: not necessarily.
Now for the long answer. First of all, yes, it might, but many people pursue love at the risk of existing relationships, and we don’t begrudge them their pursuit of happiness, even if risky. No truly good things in life are gained without risk. As a culture, we even romanticize such risky pursuits of love. I would argue that, aside from the threat of social stigma breaking them apart, they are actually less likely to break up than other couples. Assuming they were raised together, they’ve already had decades to get to know each other, most of it probably non-sexually. Imagine if a man and woman lived together for sixteen or more years, without any sex at all, before they decided to be romantically involved. We would all consider that comically conservative, and yet that is the kind of experience these siblings have had.
Even when romances do end explosively, they can still go back to normal, given time and space. There are couples that have broken up very dramatically, but after having a couple years to themselves are able to go back to being friends. Even if these siblings do ultimately break up, given all of their prior experience as siblings, the common familial relationships, etc., they should be much more likely to eventually get back to being friendly than non-related couples. They would have more motivation to.
Remember too, not all romances end explosively. Some marriages end after over a decade, on amicable terms. If a relationship ends, the destructiveness of its end is related directly to the destructiveness of the relationship itself. What destroys a relationship in such a way? Lying, abuse, lack of communication, emotional unavailability, bad conflict resolution skills, lack of respect, lack of appreciation, etc.
Since you know the couple, you should have some idea whether they have problems with any of these things in their lives. If you are their parent, then you are in a unique position to ensure that they both treat each other with respect, empathy, and honesty. You have an interest in their relationship being healthy in the long term, and you also have the power to help that happen.
Don’t assume that their relationship as siblings and their relationship as lovers are mutually exclusive. It’s a common, false assumption that they must be, but the personal testimony of people in such relationships refutes it. I doubt they fell in love because they were bad siblings, but more likely it grew out of an especially close sibling relationship. We all acknowledge that people can serve multiple roles in a relationship, being both best friends and lovers. Well, so it is that they are best friends, lovers, and siblings. Each one of those relationships strengthens the others: their relationship becomes greater than the sum of its parts.
Even if familial and romantic love were mutually exclusive, who are you to decide which of those options is best for them? So they happened to be born as siblings. Why must that chain them the rest of their lives? Maybe they will be better as lovers than as siblings. As consenting adults, they get to decide which kind of relationship makes them happiest.
Assuming you’re okay with all of the points I’ve just made, you may still have one objection: what if they have babies? This is one of the last refuges for those who can’t quite justify banning consanguinamory, but still want to. After all, what about all the stories of monster babies? Well, there are actually very few of those stories, they are an over-publicized minority, and that stereotype goes against actual scientific and historical evidence.
These siblings may already have a child. They may be pregnant. They may be planning on having a child in the future. You might have even found out about it because a pregnancy or genetic test of a child brought it to light. Once again, I must ask you to calm down, and listen carefully to what I’m about to say. The feelings you have are coming from a lot of cultural baggage and stereotyping, again. I won’t deny that the risks are higher than for the general population, but they’re not nearly as bad as you hear, and slightly elevated risks are never any reason to curtail a woman’s basic rights.
One hears an ingrained, “But it’s unnatural!” argument quite a bit. “Inbreeding” is not “unnatural,” as many would claim. Many species engage in consanguineous mating in some form or another, and it can have both positive and negative effectsdepending on the circumstances. Sometimes, species even evolve a resistance to problems from “inbreeding.” In nature, as in society, things are alwaysmore complicated than a blanket judgment can capture.

If ‘natural’ is ‘good’, what use does it serve as a moral rubric? When people say ‘homosexuality is unnatural’, they are saying ‘homosexuality is wrong’. But how does that help the discussion? ‘It’s wrong because it’s unnatural’ is the same as saying ‘it’s wrong because it’s wrong’. That doesn’t tell us why it’s wrong: it’s again ‘a description’ – a false one in this case, since […] there are 1,500 animal species that engage in homosexual behaviour. The assertion becomes a tautology. […] The entire point is to get rid of linking so tightly ‘moral’ and ‘natural’: whether something does or does not occur in nature doesn’t aid our deciding whether that act is moral. After all, wearing glasses, building hospitals and using crutches don’t occur in nature – are these to be considered ‘wrong’ based on that category?

If we should force people to only have babies with people that are distantly related from them, for eugenic reasons, then why stop at prohibiting consanguinamory? Why not forbid all sex between people of the same race? Genetic similarity within a population can still be great enough that genetic diseases are passed on – just look at Tay-SachsOf course the idea is ridiculous, but it just follows the logic of policing women’s uteri to minimize genetic disease.
I have also written about how new scientific discoveries are illuminating why, over many generations, having children with blood-relatives can have an effect on a population. It’s not what most people think, it’s not as threatening as most people think, and more importantly, we may soon be able to fix it.
Whether considering the genome, or the epigenome, a single generation can be completely inconsequential. All of the risks are population-wide risks: the chances that a random sibling couple would have a child with defects are that high, but these two siblings are not a random couple. They are a specific couple, with individual genomes. Their family history of disease is specific to their family. Those things tell you much more about their chances than some randomized study. They may, in fact, have a lower probability of defects than the general population.
Either way, we do not, as a society, agree with eugenics, and for good reason. We do not espouse the views of racists who spent decades sterilizing the poor and black in the U.S. They’ll have to care for the child, it is her body, it is their risk to take. It doesn’t matter whether you approve of it on a “massive scale” (which wouldn’t happen without society forcing people), all that matters is whether it would be okay for this specific couple.
You’re probably also worried about how the child will deal with the taboo nature of its parents’ relationship. Isn’t it better that a child grow up in a normal family? This is the kind of reasoning that punishes all sexual minorities for the bigotry of the majority. Not only do they have to deal with the derision of the masses, but now they have to give up their own children because of that derision? No enlightened person in this day and age would argue that we should take the children of same-sex couples away from them and have them raised in “normal” families. It would be barbarous, and yet there are homophobic reactionaries who argue against same-sex adoption with a similar argument.
We should never let the bigotry of others police our families. A child can learn to deal with ostracism, as long as they have a good support network at home, but no child can learn to live without experiencing love. Isn’t it better that this child grows up in an “abnormal” household that loves them dearly, than a “normal” one that doesn’t?
Here are refutations of many arguments people make against sibling consanguinamory. It’s a good addition to what I have just said. This quote from the article is especially apropos:

“There are siblings who are together right now, providing each other love, comfort, support, or their first sexual experience in a safe and reassuring environment. The biggest problem with sibling consanguinamory seems to be the prejudice and sex-negative attitudes of others. In most cases, trying to force consanguinamorous siblings apart only makes things worse. It can be a mutually beneficial way of bonding, expressing their love for each other, learning, and discovering their sexuality; it may even be a beautiful, lifelong romance. Let’s not let ignorance cause needless concern or repression.

Don’t be ashamed of changing your mind. Other people have had to walk the same intellectual and emotional journey. Don’t be ashamed that you were once wrong. Better to grow as a person than cling to terrible beliefs out of a misplaced sense of embarrassment and ego. Let yourself grow, for the sake of your child/sibling/friend. You may think you have nothing left to learn, but everyone can learn something, and everyone can teach something. This is their moment to teach you.
Here are some extra resources: