“Marrying a mother-in-law is permanently prohibited as long as a man enters into a marriage contract with her daughter, regardless of whether intimate relation has taken place or not. This is a well-known and established ruling in Islam.
Sheikh Yusuf Al-Qaradawi, states the following: “It is not permissible to marry one’s mother-in-law irrespective of whether the man has engaged in sexual intercourse with her daughter or not, and whether her daughter is divorced before consummation or not. It also makes no difference whether the wife died before consummation or not.
Referring to this, Allah Almighty says: “Forbidden unto you are your mothers, and your daughters, and your sisters, and your father’s sisters, and your mother’s sisters, and your brother’s daughters and your sister’s daughters, and your foster mothers, and your foster sisters, and your mothers-in-law, and your stepdaughters who are under your protection (born) of your women unto whom ye have gone in but if ye have not gone in unto them, then it is no sin for you (to marry their daughters) and the wives of your sons who (spring) from your own loins. And (it is
forbidden unto you) that ye should have two sisters together, except what hath already happened (of that nature) in the past. Lo! Allah is ever Forgiving, Merciful.” (An-Nisaa’: 23)
The phrase ‘and your mothers-in-law’ is so general and it includes the divorced woman whether before or after consummation as the moment the marriage contract is concluded with a certain woman, her mother becomes permanently forbidden to him whether there is sexual intercourse or not.
The issue is somewhat different in case a man enters into a marriage contract with a mother (i.e. a woman who has a grown-up daughter) without consummation. In this case, if the woman dies or is divorced before consummation, the man can marry her daughter. This ruling is also referred to in the above verse, in which Allah says: “ and your stepdaughters who are under your protection (born) of your women unto whom ye have gone in but if ye have not gone in unto them, then it is no sin for you (to marry their daughters)”
The above two rulings pertaining to marrying the mother-in-law are collectively agreed upon by all Muslim jurists, who say that ’Al-`Aqd `Ala Al-Banat Yuharrim Al-Ummahat Wa Ad-Dukhul Bil Ummahat Yuharrim Al-Banat’ (concluding the marriage contract with a woman (i.e. even without consummation) bars the person from marrying her mother, whereas consummating marriage with a certain woman does not allow the person to marry her daughter.)”