What is the expiation for the oath of the sister-in-law's husband who said to his wife's mother: "If you do not spend the night with us in the apartment today, then by my vow, your daughter will not enter your apartment," and he is regretful for his oath?

The ruling on swearing by what is forbidden takes the ruling of Dhihar. If the wife's mother stays with the husband, there is no breaking of the oath and no expiation. However, if she does not stay with him and the wife goes to her mother's house, the husband breaks his oath, and the expiation for Dhihar becomes binding upon him, which is: freeing a slave; if he does not find one, then fasting for two consecutive months; if he is unable to do that, then feeding sixty needy persons. He is forbidden from intercourse and its preliminaries until he expiates.