Is it permissible for a master to marry the sister of his female slave who was freed while he was having relations with her (Um walad)?
General Chapter
Al-Mughni
Book of Marriage
Primary text
It is impermissible for the master to marry the sister of his Um walad (a female slave who bore him a child) until her required waiting period (Istibra') has elapsed. This ruling is explicitly stated by Ahmad concerning the Um walad. The prohibition is maintained because she is considered to be observing a waiting period for him, similar to a woman observing a waiting period from a valid marriage or from sexual intercourse based on a mistaken belief (shubhah). Furthermore, there is a concern that his semen might remain in her womb, thus falling under the general prohibition of one gathering his semen in the wombs of two sisters.
Supporting text
Abu Yusuf and Muhammad ibn al-Hasan permit the marriage immediately because she is not considered his wife, nor is she in a waiting period stemming from a marriage. Zafar also prohibited it, which is considered an error, because such a marriage was permissible before she was freed, meaning it should be more permissible afterward.