Those who permitted that

Al-Zuhri said: There is no harm in a man giving a woman three divorces and then marrying her sister during her waiting period.

Al-Hasan, Sa'id bin al-Musayyib and Khilas said: If a man gives a woman three divorces while she is pregnant, there is no harm in his marrying her sister during her waiting period.

He said: Ubayd bin Nadlah used to dislike it, until it was mentioned to al-Hasan, and then it was as if he withdrew from it.