Is it permissible for a man to marry the divorcee of his father with whom the father did not consummate the marriage? Moreover, is it permissible for a father to marry the divorcee of his son with whom the son did not consummate the marriage?
Stepmothers or even foster stepmothers and all step grandmothers and their ascendants are permanently unmarriageable for their stepsons, their husbands' grandsons, and their descendants whether their marriages were consummated or not. Proof for the foregoing is the generality of the Ayah (Qur'anic verse):
And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.
Likewise, daughters-in-law are unmarriageable to their fathers-in-law or even their foster fathers-in-law, fathers of their fathers-in-law, and their ascendants whether their marriages were consummated or not. Proof for the foregoing is the generality of the Ayah which states the categories of women that one is forbidden to marry:
...the wives of your sons who (spring) from your own loins
Another proof is the Hadith in which the Prophet (peace be upon him) said,
Milk relationships are equal to blood relationships in rendering marriage unlawful [i.e. Whatever is prohibited due to blood relationship is also prohibited by milk relationship].
Regarding Allah's saying in the Ayah which is quoted above, "... Who (springs) from your own lions...," it is to exclude adopted sons; a practice that was widespread (Part No. 18; Page No. 210) in Jahiliyyah (pre-Islamic time of ignorance) but was declared Haram (prohibited) by Islam. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.