Is the marriage of the questioner to the mother of his children invalid, and must he divorce her, and what will be the fate of the children in the event of divorce, due to his committing adultery with the mother of his former wife, knowing that problems between them have resurfaced because of her refusal to allow him to marry another woman?

He who commits adultery with his wife’s mother has committed a grave evil that necessitates repentance and seeking forgiveness. Scholars have differed on whether his wife becomes forbidden to him as a result; the Hanafis and Hanbalis held that she becomes forbidden to him, while the Malikis and Shafi’is held that she does not become forbidden, and this latter opinion is the preponderant one.

As for marrying a second, third, or fourth wife, it is permissible if the man is capable of [maintaining] justice and providing for them. One must consider the benefits and harms resulting from such a marriage, especially concerning the children. If the harm outweighs the benefit, then one must refrain from a second marriage.