Being married to two wives is biblically permitted but rabinically prohibited in Ashkenazi circles since the prohibition of Rabbeinu Gershom in the tenth century. Even under biblical law however, a man may not be married to two wives who are sisters. Neither may a man divorce his wife and marry her sister. Such a marriage belongs to the category of arayot, carries the punishment of karet, and is ineffective. The only situation in which a man may marry his wife’s sister is after his wife is no longer alive. Yibum, levirate marriage, cannot be entered into with a woman whose...