The Shaykh mentioned important and useful rules regarding prohibited relatives and the prohibition of marriage through in-laws.

The Shaykh : We should say that the relatives prohibited by marriage are four, meaning four categories: the husband’s father and his son; the husband’s father, however far up the line, and his son, however far down the line, are prohibited to whom? To the wife specifically. The wife’s mother, however far up the line, and her daughter, however far down the line, are prohibited to whom? To the husband specifically. Pay attention to the rules. The wife’s mother, however far up the line, is prohibited to whom? To the husband specifically. The wife’s mother, however far up the line, is prohibited to the husband specifically. The wife’s daughter, however far down the line, is prohibited to the husband specifically. The husband’s father, however far up the line, is prohibited to whom? To the wife specifically. The husband’s son, however far down the line, is prohibited to the wife specifically. Three of these categories establish the prohibition of marriage merely upon the contract, and they are: the husband’s father, however far up the line, his son, however far down the line, and the wife’s mother, however far up the line. For these three categories, the prohibition is established merely upon the contract. Accordingly, if a man contracts marriage with a woman, her mother becomes prohibited to him. Merely upon the contract? Yes. So if he divorces her before consummation, her mother remains prohibited to him, and she is considered a mahram to him, and he may travel with her and be alone with her, because the mahram status is established merely upon the contract. Now, if a man contracts marriage with a woman and divorces her, does his father become a mahram to her?
The students: Yes.

The Shaykh : If he divorced her before consummation, yes, his father becomes a mahram to her. Does his son become a mahram to her? Yes. Accordingly, if the son of the husband wanted to marry her, we would say it is forbidden. Now, if the son of the husband wanted to marry his mother, meaning the mother of his father’s wife, is it permissible or not? Do you understand the question?
The students: Yes, it is permissible.

The Shaykh : The son of the husband wanted to marry the mother of his father’s wife. Is it permissible?
The students: It is permissible.

The Shaykh : Yes, and this is the meaning of our saying “specifically.” Pay attention to this. The word “specifically” means that it is not...