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 marriage to a man is biblically forbidden under the punishment of karet. Such a woman, in relation to such a man, is known as an ervah. The only exception to this rule is, of course, the sister-in-law herself, provided there is no other ervah relationship other than being the sister-in-law. It follows that if two brothers marry two sisters and one of the brothers dies childless, the surviving brother cannot enter into yibum with his brother’s widow. If he did so, he would violate the prohibition of being married to two sisters. The widow in this situation, being an ervah to her brother-in-law, is free to marry another man without having to conduct either yibum or chalitzah with her brother–in-law.
What if, the wife of the surviving brother dies after the death of her brother-in-law? For example, two brothers, Reuven and Shimon, are married to two sisters, Leah and Rachel, and they are all involved in a car crash from which only Shimon and Leah, Reuven’s wife, survive. Reuven dies first and he is childless. In this situation, Shimon cannot enter into a yibum marriage with Leah, his late brother’s wife, because this would result in Shimon being married to two sisters during their lifetime.
Subsequently, Rachel, Shimon’s wife, dies. At this point in time, if Shimon entered into a yibum marriage with Leah, his late brother’s wife, he would not violate the prohibition of being married to two sisters during their lifetime because Rachel is no longer alive. Nevertheless, Shimon may not, in this situation, enter into a yibum marriage with Leah.
The reason for this is because in the case of yibum the rule is: once forbidden always forbidden. That is to say, if there was an impediment to yibum at the moment of the death of the childless brother, yibum can never be entered into even after the impediment is subsequently removed.
The reason for this is grounded in the underlying ervah relationship between a woman and her brother-in-law. This forbidden ervah relationship is lifted only if there was no other impediment to the yibum marriage at the time of the late brother’s death. If any another impediment to such a marriage exists, the Torah treats the situation as though the late brother did not die childless, and accordingly the underlying forbidden ervah relationship between a woman and her brother-in-law remains.
Another reason for this continuing prohibition against yibum, even after the yibum impediment has been removed, is grounded in social concerns. It would be unfair to the widow, explains Tosafot, to allow her the freedom to marry another man, without having to perform yibum or chalitzah, only to call her back again after the yibum impediment was removed and make her undergo the chalitzah ceremony.
What if, at the moment of the late brother’s death, the surviving brother cannot effect yibum because the widow is a niddah? Or for that matter, how, asks the Chacham Zvi, can one ever enter into a yibum marriage if the “once prohibited always prohibited” rule applies? For, from the moment of the late brother’s death to the time of his burial, both the widow and the surviving brother assume the status of an onen, and an onen is both exempt and prohibited from performing the Torah’s commandments, the mitzvot. Yibum is a mitzvah.