What is the ruling on prohibiting one's wife and divorcing her due to suspicion regarding one's sister?

First: Swearing by divorce and by what is unlawful are oaths of the wicked and only swearing by Allah or an attribute of His is permissible.

Second: Whoever swears by divorce falsely has sinned and broken his oath. He must repent, and his divorce takes effect according to the majority of scholars. However, Ibn Taymiyyah holds that no divorce takes effect and no expiation for an oath is required.

Third: Whoever swears by what is unlawful falsely has sinned and broken his oath. Scholars differ on what consequences follow his oath. The preponderant view is that the swearer is questioned about his intention: if he intended divorce, then it is a divorce; if he intended ẓihār, then it is ẓihār; if he intended īlāʾ, then it is īlāʾ; and if he intended an oath, then it is an oath. He is then dealt with according to what follows from that intention.