Yevamos 1:3-4

Yevamos 1:3

Six forbidden relationships are even more stringent than the fifteen listed in mishna 1:1. Since they are married to others (and not his brother), their co-wives are permitted. These six are: (1) one’s mother, (2) his father’s wife, (3) his father’s sister, (4) a sister on his father’s side, (5) his father’s brother’s wife and (6) the wife of a brother on his father’s side.

Yevamos 1:4

Beis Shammai permit the surviving brothers to marry the co-wives but Beis Hillel prohibit it. If these co-wives performed chalitzah, Beis Shammai do not permit them to marry kohanim (as they are the equivalent of divorcees) while Beis Hillel permit it (because they performed a meaningless act). If they performed yibum, Beis Shammai permit them to subsequently marry kohanim while Beis Hillel do not (since they consider them to have had relations with someone not permitted to them). Even though Beis Hillel and Beis Shammai had some significant differences of opinion about who could marry whom, they did not hesitate to marry women from the other group, each relying on the other’s honesty in not permitting someone to marry a woman he would consider prohibited. Similarly, the members of Beis Hillel and Beis Shammai relied upon one another when it came to preparing food even though they disagreed about which utensils might be ritually pure or ritually impure.