Ibn Majah

Benefits and Issues:
Although this narration is weak, nevertheless, based on other evidences and a sahih (authentic) narration from Ibn Abbas radi Allahu anhuma, in which it is stated that (inna wat’a al-haram la yuharrim) (Irwa’ al-Ghalil: 6/287) “Fornication does not make anything lawful become unlawful,” the opinion of most scholars is that if a man commits fornication with a woman, this does not make marriage to that woman unlawful for him, nor does this unlawful act make the woman’s mother unlawful for him like a mother-in-law, nor does it make the woman’s daughter unlawful for him like a stepdaughter.

Similarly, if a man commits an illicit act with his mother-in-law or stepdaughter, this does not make his wife unlawful for him, because this relationship is not, according to the Shari‘ah, the relationship of “husband and wife,” and the aforementioned rulings pertain to the “wife.” The sin of fornication and being deserving of punishment for it is one matter, and something becoming unlawful is another matter.

For details, see: (Tafsir Ahsan al-Bayan, by Hafiz Salahuddin Yusuf rahimahullah, Surah al-Nisa, Ayah: 32)