When is a man permitted to marry his wife's sister?

It is forbidden for a man to marry a woman as long as her sister is married to him, or if she has been divorced by him with a revocable divorce and is still within her waiting period (iddah). However, if the divorce was irrevocable (ba'in) or a فسخ (annulment), then what is forbidden is the simultaneous marriage to two sisters. If the woman dies, then it is permissible to marry her sister from the moment of her death.