Yevamos, Chapter 1, Mishnah 2. This mishnah presents the practical application of the laws taught in the previous mishnah, where we learned that if the wife of a brother who died childless was a forbidden relative, she is exempt from yibum, and her co-wife is exempt along with her.
"Keitzad potros tzaroseihen":
The mishnah clarifies how the case arises in which the forbidden relative exempts her co-wife from yibum and from chalitzah: "haysah bito o achas mikol ha'arayos ha'eilu nesuah le'achiv, velo ishah acheres, umeis - keshem shebito peturah, kach tzarasah peturah" - if his daughter, or one of the forbidden relatives listed, was married to his brother, and that brother had another wife as well - she is her co-wife. If the brother died childless: just as the daughter is exempt from yibum because she is a forbidden relative, so too her co-wife is exempt, even though she herself is not a forbidden relative.
The co-wife of her co-wife:
And how do we arrive at the co-wife of her co-wife? The co-wife of his daughter, who was exempted from yibum, goes and marries a second brother among the surviving brothers, and that brother has another wife. If the second brother dies - "keshem shetzaras bito peturah, kach tzaras tzarasah peturah" - just as the co-wife of his daughter is exempt, so is the co-wife of her co-wife exempt. And so on: "afilu hen me'ah" - even if the chain extends through a hundred brothers, each one marrying the co-wife of the one before him, the exemption carries through to all of them.
"Keitzad im meisu tzaroseihen mutaros":
This explains the law taught at the end of the previous mishnah, that if the forbidden relative died, her co-wives are permitted: "haysah bito o achas mikol ha'arayos nesuah le'achiv, velo ishah acheres, meisah bito o nisgarshah, ve'achar kach meis achiv - tzarasah muteres" - if his daughter, or one of the forbidden relatives, was married to his brother, and that brother had another wife, and his daughter died or was divorced, and afterward his brother died, her co-wife is permitted. Since at the moment the brother died the two were not co-wives of each other, for the forbidden relative had already died or been divorced beforehand, there is no exemption of the co-wife of a forbidden relative here, and the co-wife is permitted.
"Kol hayecholah lema'en":
The mishnah concludes with the law of a minor girl whose betrothal is only rabbinic, and who, as long as she is a minor, has the power to refuse and annul her marriage, as was mentioned in the previous mishnah. The case is as follows:
The forbidden relative: the daughter of the surviving brother, who was married to the brother who died with a betrothal that was only rabbinic, and who did not refuse. By Torah law there is no marriage here at all, and therefore she is not a full forbidden relative in relation to the brother who died.
The co-wife: an adult woman whose marriage is valid by Torah law, so that by Torah law she is obligated in yibum or chalitzah, since she is not truly the co-wife of a forbidden relative.
Therefore we learned: "kol hayecholah lema'en velo mi'anah - tzarasah choletzes velo misyabemes" - any girl who is able to refuse and did not refuse, her co-wife performs chalitzah and does not undergo yibum. The co-wife is not permitted to undergo yibum, because on account of the rabbinic betrothal she appears to be the co-wife of a forbidden relative. On the other hand, she is not exempted entirely, for the co-wife of a full forbidden relative does not even require chalitzah, whereas here, since by Torah law this is not the co-wife of a forbidden relative, the bond remains upon her and she performs chalitzah.
In summary: in this mishnah we learned how the law of the co-wife of a forbidden relative is applied - how the forbidden relative exempts her co-wife and even the co-wife of her co-wife, up to a hundred; when the forbidden relative died or was divorced before the brother's death and the co-wife is permitted; and the law of the minor whose betrothal is rabbinic and who did not refuse, whose co-wife performs chalitzah and does not undergo yibum.