If an adult man commits fornication or adultery with an adult woman, whom among her relatives become forbidden for him?

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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If a person commits fornication or adultery with an adult woman, her mother and daughter will become forbidden for him, and she will be forbidden for his father and his son, exactly as in permissible marriage. Besides, he is not permitted to marry her sister until she [the woman with whom he committed fornication or adultery] has one monthly period to make sure that she is not pregnant, or until she gives birth if she became pregnant. The same thing applies if he wants to marry her maternal or paternal aunt. In case one committed fornication or adultery with his sister in-law, Imaam Ahmad said: 'He should refrain from sexual intercourse with his wife until she has three menses.'

The forbidden sexual relations makes what is lawful forbidden, as discussed, and this is a matter which is reported by Imraan Ibn Husayn, Al-Hasan, ‘Attaa’, Taawoos, Mujaahid, Ash-Shu’bi, An-Nakha’i, and Ath-Thawri and this is also the view of the Hanafi and Hanbali schools. However, many other scholars adopted the opposite opinion as clarified in Fatwa 86518, so please refer to it. 

Allaah Knows best.