What is the ruling regarding a man who swore to divorce his wife threefold – by making her unlawful to him – if her sister were to enter his home? What are the consequences if she does enter? And is he required to offer expiation for breaking an oath?

The majority of scholars hold that swearing an oath by divorce results in divorce if the oath is broken, and that uttering the triple divorce pronouncement counts as three divorces. This is the established ruling. Therefore, if the aforementioned woman enters your house, your divorce will be effective as a triple divorce, and your wife will be irrevocably separated from you (baynunah kubra).

However, Sheikh Al-Islam Ibn Taymiyyah is of the view that swearing an oath by divorce, when the intention is not to make the divorce conditional but rather to threaten or emphasize, is treated like an oath by Allah. In this case, if the oath is broken, an expiation for an oath (kaffarat yamin) is required, and no divorce occurs. He also holds that pronouncing divorce three times counts as a single divorce. Accordingly, if your intention was to prevent the woman and not to effect a divorce, then you may permit her to enter and perform a single expiation for an oath, which is feeding ten poor persons or clothing them, or fasting three days if you cannot find them.

We should note that swearing an oath by divorce is among the oaths of the dissolute, and the legislated oath is swearing by Allah the Almighty.