[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.