So, I’ve been searching for a solid answer for this for some while, and it ends up that I can’t find anything that’s consistent or that well researched. I’m familiar with the rudiments of biology that go into the situation I’ll expound upon below, but don’t quite have certainty about the finer points. So I figure, let me take a stab out into the Tumblr verse to see what you all have to say.

In my extended family, I have a set of step-cousins that I’m not at all close with (My maternal grandmother’s second husband’s biological grandkids.). Two of the girls of this line have had children with two men. At first, the story went that the guys were brothers. That’s not all that uncommon, as I have about three instances of it on my father’s side of the family two or three generations back. Basically, you end up with children that are double first cousins. However, what we found out later was that the two brothers are identical twins. This was sort of an interesting talking point I had with my family as it presents a curious situation. Biologically “identical” men having children with two sisters. Does the twin aspect actually affect the genetics of the situation? Do we end up with children that are both first cousins AND half-siblings!?

Now I have done a basic google search and haven’t really found anything of worth. But what usually happens is that people conflate the genetic aspect of the situation with the social aspect. That is, there are two men and two women, children come from one man and one woman, so no they’re not siblings. And I follow that logic and, socio-culturally, it’s true for most mainstream societies. (If this were Fraternal Polyandrous society this would be a whole other thing!) But, that’s basically reductive, bringing the argument down to 1+1=2, when that’s not the question we’re actually tackling. 

I do understand that we’re finding out that identical twins are less genetically identical then originally thought, but more identical than dizygotic twins, which leaves us with this sticky situation. My gut says that monozygotic twins would more or less be of a genetic profile that is identical within an allowable +/- based on environmental factors during development, and as such would result in children being genetically half-siblings, even if that’s not the way we’d socially recognize it.

What say you? Are the children of identical twins half-siblings? Just first cousins? Or something somewhere in-between?