Concept:Islamic law defines permanent and temporary marriage prohibitions. A man may marry his deceased wife's sister, but not while his wife is alive.Explanation:Islam forbids combining two sisters in marriage at the same time. After the death of his wife, this restriction no longer applies. Therefore, marrying the late wife's sister is allowed. Foster relatives, such as a foster sister or foster mother, are permanently prohibited. A stepmother, i.e., a late father's wife, is also permanently prohibited.Answer:Late wife's sister.