The famous ???”? which says that they only kept the ???? inside ??? ????? will not even answer the question, because they married their siblings while living in ??? ?????.
The ???”? explains that they kept the ???? even outside ??? ?????, and the way they married (I think he is talking about ???? marrying two sisters) was because of the principle ?? ??????? ???? ????? ??? – A ger who enters Judaism from any other religion loses any relations he had and is considered a newborn child. The ???? uses this to say that ???? can marry their siblings (at least ?????????).
The ???? only kept the ???? using the concept of ????, since they as ??? ?? were not ?????. Since their whole ???? was as ????, they only kept the ????? inasmuch as their ????, which therefore told them they were not related to their siblings.
I once heard a ??? that since ????? before ??? ???? was only maternal, the ????? would marry the relations that were only paternal, and they DIDN’T marry their own twins, but rather each others twins. (This is how ????? married ????; see ???? ????? ?? ????? in ???? ????)
What I wonder is: If each ??? was born with a twin, what happened with ???? and ????? Was ???? born with a twin girl (because she was originally intended to turn out male) and was ???? born was a twin boy?