Hazrat Zuhri says that there is no harm if a man divorces a woman three times and, during her waiting period (‘iddah), marries her sister.

Hasan, Hazrat Saeed bin Musayyib, and Hazrat Khallas state that if a person gives three divorces to a woman and that woman is pregnant, then there is no harm in marrying her sister during her waiting period. Ubaid bin Nazeelah considered this to be disliked. When this matter was mentioned to Hasan, it seemed that he also disliked it.

17598 - He said: And Ubayd ibn (Nadlah) (1) used to dislike it until it was mentioned to Al-Hasan, and it was as if he withdrew from it.