How common is it for step siblings to marry?

In this article, we will answer the following question: How common is it for step siblings to marry? We will talk about the moral implications of marrying your step-sibling, the Westermarck effect and about Genetic sexual attraction. 

How common is it for step siblings to marry?

Many cultural taboos are created around genetic problems. There is nothing biologically wrong with two people who were created by completely different parent pairs being attracted to one another.

Social issues could be an entirely different thing. The longer you may have lived under the same roof, the more strange you may be seen by outsiders. I would say that this is entirely curable by moving to a different community, but without knowing how old and independent the parties are, it is difficult to say if this is practical.

So, as with many issues with which people struggle, it may depend on circumstances, but there is nothing morally repugnant about the situation, you simply have to ask what are the consequences in your community and your family?

Let’s take an example. If your stepsister is your mother’s daughter with another father, she still has half your blood. You are both brothers. I don’t know why if the children are children of the father and of a different mother they are called siblings and if they are children of the mother and of a different father they are called stepbrothers.

Now: if they are two different families, that is, that your stepsister was the daughter of your stepfather or your stepmother but you do not have any blood relationship, I do not know how legal things will go with that.

The blood issue is because children inherit diseases and acquire others. It is like half-inc**t.

Can you marry a step sibling?

Yes, you can marry a step sibling, because stepsiblings are not blood relatives, which means that there is no risk of any genetic disorders carrying through them to their children. Step Siblings are legally free to marry each other and in the US there are no state laws that prohibit marriage.

Is it illegal for step siblings to marry?

No, it is not illegal for step siblings to marry, but it is not seen in a positive light by society. It is illegal for siblings to marry because in this case the chances of genetic disorders in their children, but this is not the case with step siblings. Step siblings are related through their parents’ marriage, and not by blood, which is why it is legal for them to get married.

Why do most people feel the rejection of a sibling relationship, even if it is consensual? 

The answer is inc**t taboo. In the vast majority of cultures, sexual relations between relatives with different degrees of consanguinity – parents with sons and daughters, brothers among themselves and, to a lesser extent, cousins – are not socially accepted. Sometimes they are also legally prohibited.

One explanation is the rule of exogamy: in the primitive tribes, young people looked for their partner outside the group – in some societies women left and in others, men – and these marital ties favoured alliances with other lineages, which strengthened the family nucleus and reduced the possibility of confrontations with neighbours, but there is also a biological reason. 

When human beings are paired with their relatives over several generations, the risk of inherited diseases increases; individuals become less fertile and reproduce less. In other words, evolution rewards the dispersion of genetic inheritance and punishes its concentration.

An argument that reinforces the biological interpretation is that also some animal species avoid mating with members of their family. In species such as chimpanzees or spider monkeys, females must leave their natal groups when they reach sexual maturity and search for a new one. In the cappuccinos or the macaques, it is the opposite, it is the males that must leave the group. 

An adaptive mechanism that guarantees that this rule is met is the development of sexual rejection towards the people with whom we live; The so-called Westermarck effect is established in early childhood, but, according to some scientists, it would also explain the decrease in desire in long-term marriages.

The Westermarck effect: lack of desire for childhood friends

Many people are interested in knowing what characteristics and styles of behaviour enhance personal attractiveness, but fewer also try to know things about the factors that kill any possibility of attraction.

That is why it is not surprising that so little is known about the Westermarck effect, a hypothetical psychological phenomenon according to which human beings are predisposed not to feel sexual desire towards the people with whom we interact continuously during our early childhood, independently whether they are related or not.

Why might this curious trend occur? The explanatory proposals that many researchers are considering to solve the unknown of the Westermarck effect have to do with the phenomenon of inc**t.

The probability

Edvard Alexander Westermarck was a Finnish anthropologist born in the mid-19th century known for his theories about marriage, exogamy, and inc**t. 

Regarding the latter, Westermarck proposed the idea that avoidance of inc**t is the product of natural selection. For him, avoiding reproduction between family members would be part of an adaptive mechanism that we carry in our genes and that would have spread among the population due to the advantage of this behaviour in evolutionary terms.

Since offspring from inc**t can have serious health problems, selection would have carved out in our genetics a mechanism for us to dislike it, which in itself would be an adaptive advantage.

Ultimately, Westermarck believed that natural selection has shaped the sexual tendencies of our entire species by preventing relationships between close relatives.

Suppressing sexual attraction to avoid inc**t

But how would you do natural selection to promote inc**t avoidance behaviours? After all, there is no trait by which we can recognize brothers and sisters with the naked eye. 

According to Westermarck, evolution has decided to pull statistics to create a mechanism of aversion between relatives. As people who see each other daily and belong to the same environment during the first years of life have many possibilities of being related, the criterion used to suppress sexual attraction is the existence or not of proximity during childhood.

This predisposition to not be attracted to the people with whom we come in contact periodically during the first moments of our lives would be genetically based and would represent an evolutionary advantage; but as a result of this, we would also have no sexual interest in old childhood friendships, for example.

Genetic sexual attraction

The curious thing is that the scientific literature describes that imprint that leads us to avoid relationships with close relatives or childhood friends, but also an apparently contradictory phenomenon: genetic sexual attraction, by which members of a family who grow up separated (step-siblings for example) can feel attracted when they are older

This syndrome has become known in recent years due to the reunions between relatives who had lived in different homes as a result of adoption processes.

Patrick Stübing and Susan Karolevski grew up separated because he was given up for adoption at the age of 3 in Leipzig (Germany). Two decades later, he met his biological family and began a relationship with his 16-year-old sister, who suffered from slight mental retardation. 

They had four children: two of them have severe intellectual disabilities, one was born with congenital heart disease and the fourth is normal. As inc**t is illegal in Germany, Stübing was imprisoned and the custody of the three older children was taken from them. The case was very controversial – a sector of society asked to break “the last taboo” – but in 2012 the Strasbourg Court ruled in favour of German Justice.

There is a group of inherited diseases, that to manifest require that both copies of the gene are mutated. In this case, if the parents are relatives, their descendants have a 50% chance of being carriers of the disease and a 25% chance of suffering from it.

Although the probabilities that the children suffer anomalies depend on the degree of consanguinity, it is calculated that they are close to twice that of people without a relationship. Phenylketonuria, cystic fibrosis or porphyria are some ailments of this group.

FAQ on How common is it for step siblings to marry?

Do people marry their step-siblings?

Generally, people can marry their step-siblings, unless the law in their country prohibits it. If step-siblings are not related by blood, they can get married and even have children. However, marriage between people who are related by blood is illegal. 

Is marrying your step-sibling illegal?

Marrying your step-sibling, if you are not related by blood, is not illegal. Otherwise, marriage between people who are related by blood (sibling, parent, grandparent, child, grandchild, stepparent, grandparents’ spouse, spouse’s child, spouse’s grandchild, sibling’s child or parent’s sibling) is punished by law in most countries of the world.

Is it normal to be attracted to family members?

It is not normal to be attracted to family members, however, there are enough cases out there to prove that this happens. There is even a term for this, called Genetic Sexual Attraction, or GSA. 

Why don’t we find our siblings attractive?

We do not find our siblings attractive because we grew up too close to them. The constant closeness leads to saturation of stimuli and too much familiarity between brother and sister, which kills any attraction. This can be seen even in people who are not relatives, but who grow up in the same house.

What is it called when you are attracted to your sibling?

When you are attracted to your sibling, you may speak of Genetic sexual attraction. GSA means that a strong sexual attraction may develop between close blood relatives, but only when the two first meet as adults. In other cases, we speak about inc**t. 

Is it illegal to sleep with your cousin?

In most countries, yes it is illegal to sleep with your cousin, parent, grandparent, sibling. 


In this article, we answered the following question: How common is it for step siblings to marry? We talked about the moral implications of marrying your step-sibling, the Westermarck effect and about Genetic sexual attraction. 

As a summary, there are enough cases of romantic and passionate love between brother and sister to convince us that sibling inc**t can happen. There is nothing biologically wrong with two people who were created by completely different parent pairs being attracted to one another.

However, you should consider that social issues could be an entirely different thing. The longer you may have lived under the same roof, the more strange you may be seen by outsiders.

If you have any other questions or comments on the content, please let us know!


DeYoung, M., & Lowry, J. A. (1992). Traumatic bonding: Clinical implications in inc**t. Child Welfare: Journal of Policy, Practice, and Program, 71(2), 165–175.

Pendergrast, M. (1995). Victims of memory: Inc**t accusations and shattered lives. 

“Inc**t”. Author: Julia Máxima Uriarte. To: Last edition: March 10, 2020. – Is it Illegal for Step-Siblings to Marry? – In What States Is It Illegal to Marry Step-Siblings?