Is it permissible for a married man—whose wife has died or been divorced—to marry her sister before he marries her?

A man is not subject to an 'iddah (waiting period) in Islamic law or linguistically. However, the term 'iddah may be applied to him metaphorically in certain situations, such as when he divorces his wife and wishes to marry a woman whom he is forbidden to combine in marriage with her (like her sister or paternal aunt). Or, if he has four wives and divorces one of them, he is not permitted to marry until the divorced wife's 'iddah has concluded. This includes revocable divorce by consensus, and irrevocable divorce according to the Hanafis and Hanbalis. However, in the event of his wife's death, he is immediately permitted to marry her sister or any woman whom he was forbidden to combine with her, as the marital bond is severed by death.