[If] three brothers were married to three strangers and one of them died, and [then] the second performed maamar with her and [also] died —- they require chalitzah, but may not be taken in yibum, as it is stated (Deut. 25:5): “. . . and one of them dies . . . her husband's brother shall come to her” —- [only a widow] who is attached by one brother's bond [is subject to yibum], not [a widow] who is attached by two brothers' bonds. R' Shimon says: He may perform yibum with whichever one he wishes, but he must [then] perform chalitzah with the other.  [If] two brothers were married to two sisters, and one of them died, and then the second one's wife died —- she is forbidden to him forever, because she was once forbidden to him.