Q: What is your opinion concerning
the ruling on marrying a grandson's ex-wife after she finishes her `Iddah (woman's prescribed waiting period after divorce or widowhood)? Is it lawful or unlawful for that woman to marry that man?
If it is unlawful, what is the evidence? May Allah reward you.
It is impermissible for a man to marry a woman who was previously married to his son or grandson, whether that relation is by marriage or a foster relation. This is according to Allah's saying when He mentions the women who are unlawful to marry,
...the wives of your sons who (spring) from your own loins
If a man divorces his wife or dies, it is impermissible for her to marry his father, or paternal or maternal grandfather, as all grandfathers take the same ruling concerning this, due to the generality of the Ayah. Also, Allah's saying about Yusuf (Joseph, peace be upon him) when he said,
"And I have followed the religion of my fathers, - Ibrâhîm (Abraham), Ishâq (Isaac) and Ya‘qûb (Jacob) [عليهم السلام]
(Part No. 18; Page No. 211) He is Yusuf ibn Ya`qub ibn Ishaq ibn Ibrahim (peace be upon them). Allah's saying,
...from your own loins
excludes adopted sons, as they used to adopt children during the time of Jahiliyyah (pre-Islamic time of ignorance), so Allah (Exalted be He) prohibited this in His saying,
...nor has He made your adopted sons your real sons. That is but your saying with your mouths. But Allâh says the truth, and He guides to the (Right) Way.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.