Yevamos 28a-29a
The rabbinic prohibition of Achos Zekukaso, levirate bonds between two widows who are sisters and their surviving brothers-in-law \u2022 If the prohibition was violated, may the new couple stay married? \u2022 Various cases where widows who are sisters may both have levirate marriages \u2022 When both are necessary, why must a bill of divorce be given before Chalitzah is performed?