1) I saw R’ Chaim Kanievsky (??? ????) brings a ???? that says that ??? and ??? were not sisters from the mother, they were only paternally related. He says that therefore ???? could marry two sisters, because before ??? ???? there was no such thing as ???? ?? ???.
2) The ???”? in ???? ???? says that the ???? were not ????? to keep the ????, and if they wanted to keep it, they had to keep it as if they were ????, and ???? have a din of ?? ???????? ???? ????? ??? and therefore in-as-much as he was ?????, they were not related to each other.
3) The famous ???”? – they only kept the ???? in ??? ?????. (which is a bit shver in ???”? ????? ?????)
4) Before ??? ????, there was no such thing as ??????? (See ???”? ??? ? ????? ??’ ?), and therefore it wasn’t possible to become assur with ???? ???. (Even if they kept the ????, there was no ???????).
See ????”? ????? ?”? who proves from ???? that there was no ????? which come through marriage before ??? ???? except for ????? ???.