What is the relationship of these two siblings?

Assuming the twins are identical twins, biologically/genetically the children are full siblings, but in a practical sense they are half siblings. Also, they are half cousins if there is such a thing.

If they are living together in a polygamous relationship, they could both be called mother. If the twins are not on speaking terms because one stole the others boyfriend, they probably don’t talk to each other at all. There are a whole lot of possible scenarios in between those extremes too. Being a mother has more to do with the nurturing of a child than the biological creation of one. IMO

Legally they are half siblings.

@JLeslie They are double cousins, not half cousins. Double cousins share both sets of grandparents.

The word for this relationship is: sick.

@keobooks I have never heard of that term. Thanks.

@SadieMartinPaul Most unusual, I grant you, but why do you describe it as sick?

From looking into my family genealogy I have found that in the smaller communities of the past, where good partners are sometimes scarce and families intermarry frequently, it is not uncommon for siblings to marry the same spouse. I have not found a situation where they did it at the same time but upon the death of one the widow/widower is a viable option.

@Adagio The thought of a man having sexual relations with both twin sisters puts my Creep-o-Meter on a high setting.

I think @rojo brings up a good point: In olden times families intermarried much more often and it wasn’t creepy or weird. It was normal.

For some reason, one of my husband’s aunts gets really offended by the term “double cousin” because she says it sounds inbred. But her two sons married two sisters, so all their kids are double cousins and they really do look and act like siblings (they are also neighbors) I think she’s being a bit sensitive about it. It’s obvious to anyone that the kids are closer than cousins but not quite siblings. Not just by blood—but socially as well.

I knew two sisters who had married two brothers. Their children shared the same DNA pool, and they were genetic siblings. I’d never heard the term “double cousins,” so I just thought of them as genetically identical.

But, that’s a different scenario than one man fathering children with identical twin sisters. That concept brings out the “Ew!” in me.

@SadieMartinPaul I’ve assumed all parties are happy with the arrangement, in which case I don’t have a problem with it.