Four brothers, two of whom were married to two sisters, [and] those who were married to the sisters [both] died, [both of] these [sisters] require chalitzah, but they may not be taken in yibum. And if [the brothers] went ahead and ma-rried [the sis-ters], they must divorce [them]. R' Eliezer says in the name of Beis Shammai: They may keep them; but Beis Hillel say: They must divorce them.