[If there were] four brothers, [and] two of them were married to two sisters, and those married to the sisters died —- they require chalitzah but may not be taken in yibum; if they went ahead and married [them], they must divorce [them]. R' Eliezer says: Beis Shammai say they may keep [them], but Beis Hillel say they must divorce [them].

[If] one of them was forbidden to one [of the brothers] by the ervah prohibition, he is forbidden to her but permitted to her sister, while the second [brother] is forbidden to both. [If it was] a prohibition by commandment or because of sanctity, she requires chalitzah, but may not be taken in yibum.