Question 1: If a person divorces his wife and she is still in her `Iddah (woman’s prescribed waiting period after divorce or widowhood), is it permissible for him to marry her sister? If his wife dies, is it permissible to marry her sister shortly after? May Allaah benefit us and you!
Answer: If a person divorces his wife, it is not permissible for him to marry her sister or maternal or paternal aunt unless the `Iddah is over if the divorce is revocable as agreed by all Muslim scholars. If the divorce is revocable, then she is still his wife as long as the `Iddah is not over. But if it is irrevocable divorce, such as being the third divorce or Kul` (divorce initiated by a wife for a consideration), this is a controversial matter among scholars… read more here.