Yevamos 2:7-8

Yevamos 2:7

If two men married two sisters but they don’t know who married whom, each man must give a divorce to each woman. If the two men die and each of them leaves a brother, each brother must perform chalitzah with each woman. If one deceased husband left one brother and the other left two brothers, the one brother must perform chalitzah with each woman; of the two brothers, one must perform chalitzah with the first woman, after which the other has the option to perform yibum with the other woman. If the two brothers marry the two women, they may remain married even though this was not the appropriate course of action. If each deceased husband left two brothers, one brother of the first deceased performs chalitzah with one woman and one brother of the other deceased performs chalitzah with the other woman; after this, each of the other two brothers has the option to perform yibum with the woman who performed chalitzah with the other one’s brother. If both brothers of one of the deceased jumped the gun and both performed chalitzah, then the two brothers of the other deceased may not both perform yibum. Rather, one must perform chalitzah, after which the other has the option to perform yibum. If they married the two widows, they may remain married even though this was not the appropriate course of action.

Yevamos 2:8

It is a mitzvah for the eldest surviving brother to perform yibum but if a younger brother precedes him and marries the widow, it is valid. If a man has been accused of having an inappropriate relationship with a Canaanite maidservant and she was subsequently freed, or with a non-Jewish woman and she subsequently converted, he may not marry her (because people will assume that the rumors were true). If he did marry her, they may remain married even though this was not the appropriate course of action. If a man is suspected of having an inappropriate relationship with a married woman, for which she must divorce her husband, he may not marry her. If he did marry her, they may not remain married.