[If] one of them was forbidden to one [brother] by the ervah prohibition, and the second one was forbidden to the other [brother] by the ervah prohibition —- the one forbidden to this [brother] is permitted to the other [brother], while the one forbidden to that [brother] is permitted to this [brother]. This is what was meant by the statement: [If] her sister is her [fellow] yevamah she may either perform chalitzah or be taken in yibum.

[If there were] three brothers, [and] two of them were married to two sisters, or [to] a woman and her daughter, or daughter's daughter, or son's daughter -— they require chalitzah but may not be taken in yibum. R' Shimon, however, releases [them]. [If] one of them was forbidden to him by the ervah prohibition, he is forbidden to her but permitted to her sister. [If it was] a prohibition by commandment or because of sanctity, she requires chalitzah but may not be taken in yibum.