What is the ruling on one who swore to his wife by triple divorce that she is forbidden to him like his mother or sister if a certain matter did not occur, and that matter did not occur?

This oath of triple divorce does not result in a breach (ḥanith). Rather, ẓihār (a form of divorce by injurious assimilation) occurs upon the fulfillment of the condition upon which it is suspended. As Sheikh 'Ulaysh said regarding one who states, "Upon me be triple divorce if you speak to Zayd, then you are divorced," the answer is that one divorce becomes binding upon him if he did not intend more, and ẓihār occurs through the declaration of taḥrīm (making something unlawful), based on the statement of Ibn Qudāmah al-Ḥanbalī: "If he says: 'You are unlawful like my mother,' then it is an explicit declaration of ẓihār, because it does not bear any meaning other than unlawfulness." The majority of scholars hold that a suspended ẓihār takes effect upon the fulfillment of the condition, while Ibn Taymiyyah believes its ruling is like an oath by Allah if the intention was to affirm, prevent, or encourage. The fatwa issued is according to the opinion of the majority. If the condition is fulfilled, ẓihār occurs, and it is not permissible to have marital relations with the wife before expiating the ẓihār mentioned in Sūrat al-Mujādalah, unless the husband or wife dies or he divorces her, in which case there is no obligation upon him. The husband should avoid swearing by divorce and unlawfulness. The lawful oath is to swear by Allah the Almighty.