Yevamos 1:1-2

Yevamos 1:1

There are 15 women who exempt their co-wives and their co-wives’ co-wives from yibum and chalitzah (levirate marriage and its release ceremony); this exemption applies to the co-wives of co-wives ad infinitum. These 15 women are (1) if the deceased brother was married to the surviving brother’s daughter, (2) his daughter’s daughter, (3) his son’s daughter, (4) his wife’s daughter, (5) her son’s daughter, (6) her daughter’s daughter, (7) his mother-in-law, (8) his mother-in-law’s mother, (9) his father-in-law’s mother, (10) his sister on his mother’s side, (11) his mother’s sister, (12) his wife’s sister, (13) the wife of a brother on his mother’s side, (14) the wife of a deceased brother who was not alive at the same time as the surviving brother and (15) his daughter-in-law. In all of these cases, if the disqualifying woman died, invalidated the marriage (in the case of a minor orphan girl who was married off by relatives), divorced or was found to be incapable of bearing children, then her co-wives become permitted. The scenarios of being found incapable of bearing children and a minor girl refusing a marriage do not apply to the cases of a mother-in-law, a mother-in-law’s mother and a father-in-law’s mother (all of whom are obviously adults and have already borne children).

Yevamos 1:2

Here’s how these women exempt their co-wives: If the surviving brother’s daughter – or any of these other forbidden relationships – was married to the deceased brother, and the deceased brother had another wife in addition to her, just as the surviving brother’s daughter is exempt from yibum and chalitzah, so is the other wife. If the co-wife married yet another brother, who also had another wife, and then he died, just as his daughter’s co-wife is exempt, so is that co-wife’s co-wife with the other brother. This process would continue to exempt co-wives even 100 times. If the surviving brother’s daughter – or any of these other forbidden relationships – was married to a brother who had another wife and the daughter died or was divorced before the brother died, then her co-wives are permitted. If a minor who could have refused the marriage did not do so, she only performs chalitzah (the release ceremony) and not yibum (levirate marriage) because the marriage is rabbinic in nature.