When does it become permissible for a man to marry his wife's sister?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

It is unlawful for a man to marry a woman so long as her sister is in his wedlock or is not irrevocably divorced from him and is still in the period of her ‘Iddah, and there is consensus among scholars about this. But scholars held different opinions about this when the divorce is irrevocable or the marriage contract is annulled. Some are of the opinion that it is unlawful for him to marry his wife's sister during the period of her ‘Iddah; that is the opinion of Sa‘eed ibn Al-Musayyib, Mujaahid, An-Nakha‘i, Ath-Thawri, Ahmad and the Hanafi scholars of Fiqh. It was narrated on the authority of ‘Ali, Ibn ‘Abbaas and Zayd ibn Thaabit. But Al-Qaasim ibn Muhammad, ‘Urwah, Ibn Abu Layla, Maalik, Ash-Shaafi‘i, Abu Thawr, Abu ’Ubayd and Ibn Al-Munthir, are of the opinion that he has the right to marry her. According to them, what is unlawful is to combine both of them in wedlock at the same time, and the irrevocably divorced woman is not in his wedlock, and since she is irrevocably divorced she is similar to the divorced woman before consummating the marriage. That is the predominant opinion. So, it should not be abandoned.

In case his wife dies, it becomes permissible for him to marry her sister from the very moment of his wife’s death, according to the scholars’ consensus.