Brothers and sisters

In the article:

Comments that children of two families where fathers and mothers are both identical twins the siblings are not brothers and sisters but cousins.

I agree but want to state that this is trivially true if you think about adoption. Adopted child is a brother or sister to biological siblings or several adopted childs are brothers and sisters.

So being a brother or sister is not about genetics but about living and groving in same household. That’s why humans and chimpansees are cousins not brothers and sisters.

If a pair of identical twin men marry a pair of identical twin women then all of their children will be genetically siblings but socially the children of one married set will be siblings and socially cousins to the siblings of the other married pair.

I really don’t get why this is so confusing to people.

On the flip side, if a married pair adopts unrelated children then genetically their adopted children aren’t related to the parents, but socially they are their parents’ children.

In sum: genetic and social relatives are not always the same thing, although they often overlap.

I think you are adding to the confusion here. You want to introduce the terminology “social siblings” vs “genetic siblings”, but that’s not used in genetics.

In my experience, the terms “sibling” and “cousin” refer solely to familial relationships, not genetics.

The genetic concept is the coefficient of relationship, r.

So in the case of the children of two families where the fathers and mothers who are two pairs of identical twins: we would still call them cousins, but note they have the same genetic coefficient of relationship (r=0.5) that siblings usually do.

ETA: to Broomstick, I see that googling “genetic siblings” does get some hits, so I apologize - it does appear the term is sometimes colloquially used. But I’ll stick to my claim that it’s not the usual terminology in genetics (I’m an evolutionary biologist).

I’m totally OK with geneticists using different terminology than the rest of us.

I suspect that geneticists are primarily concerned with (surprise!) the genetics. Some of the rest of us put the social/cultural relationships first. Perhaps it is akin to the way that to a geneticist “clone” refers to any number of individuals sharing a genotype and the rest of us might refer to “identical twins” or “identical triplet” or “clone” (if we’re not specifying them being of near identical age or specific number) or even “monozygotic twins” if we have just enough knowledge to be dangerous. :wink:

So… there’s the specific jargon of genetic science, and then there’s the rest of us using language in a slightly different manner.

Double first cousins. My Daddy had some. His Dad and Uncle married a pair of twin sisters. It’s never been confusing at our reunions.

You’re correct, but double first cousins would also be if any pair of siblings married another pair of siblings, they don’t have to be twins.

Have you met people? There are grown adults who don’t know the difference between step siblings and half siblings, and some who don’t realize boy/girl twins can’t be identical. This is a more difficult concept of either of those things.