Chapter 8, Mishnah 6. In the previous Mishnah we learned that a woman who remarried on the strength of an invalid get is penalized severely: she must leave both husbands, any child is a mamzer from both of them, and she forfeits a whole series of financial rights. Our Mishnah and the next one present additional cases in which the same penalties are imposed.
Background: A co-wife of a forbidden relative is exempt from yibum:
Our Mishnah is built on the opening Mishnah of Maseches Yevamos. There we learned that when a man dies childless, his wife comes before the yavam for yibum. If the widow is a forbidden relative to the yavam (for example, she is his mother-in-law), she is exempt from yibum, since she cannot violate a prohibition of arayos in order to fulfill the mitzvah of yibum. The Mishnah teaches that not only is the forbidden relative herself exempt, but so is her co-wife. In other words, if the deceased had two wives, the second wife is also exempt from yibum, at least according to Beis Hillel.
The case in our Mishnah:
The deceased had two wives, and one of them was a forbidden relative to the yavam. On that basis her co-wife was also exempt from yibum, and relying on this exemption the co-wife went and married another man. Afterward it emerged that the forbidden relative was an aylonis, a woman incapable of bearing children. An aylonis is not subject to yibum at all, and since the mitzvah of yibum never applied to her in the first place, she does not exempt her co-wife. It turns out that the co-wife was in fact obligated in yibum, and she married an outsider unlawfully.
In the words of the Mishnah: "Kol ha'arayos she'amru tzaroseihen mutaros" - all those forbidden relatives concerning whom the Sages said that their co-wives are permitted to marry anyone and are not bound to the yavam. "Halchu hatzaros hallalu venisu" - these co-wives went and married, relying on that exemption. "Venimtze'u eilu aylonios" - and it emerged that the forbidden relative was an aylonis, who is not included in the law of yibum at all. "Teitzei mizeh umizeh" - the co-wife must leave both husbands, the one she married and the yavam, "vechol hadrachim ha'eilu bah" - and all the penalties listed in the previous Mishnah apply to her as well.
(It is worth noting that some explain the exemption of the aylonis as stemming from the fact that her marriage was a mistaken transaction, since the husband did not know that she was incapable of bearing children, so there was no marriage at all. There are differing views on this. In any case, according to every approach it turns out retroactively that the exemption the co-wife relied upon was void, and she is married in violation of the prohibition.)
The view of Rabbi Akiva:
The Gemara explains that this Mishnah follows the view of Rabbi Akiva regarding the status of a mamzer. The question is which forbidden marriages produce a mamzer:
The halachah, which does not follow Rabbi Akiva: A mamzer is produced only from a union in which kiddushin does not take effect at all, such as a woman who is a forbidden relative, to whom a man cannot be married in any way.
According to Rabbi Akiva: Even chayavei lavin, an ordinary Torah prohibition, produce a mamzer. So too in our case: a yevamah who is bound for yibum may not marry anyone other than the yavam, and this is the prohibition of "yevamah lashuk," which is not a prohibition of arayos but only a regular lav, and nevertheless the child is a mamzer.
It emerges that only according to Rabbi Akiva is the co-wife's marriage severe enough to warrant imposing all these penalties upon her. But according to those who disagree with him, the violation is not severe enough, and we would not impose all those penalties on a co-wife who married unlawfully.
In summary: In this Mishnah we learned that a co-wife of a forbidden relative who married on the strength of the yibum exemption, and it then emerged that the forbidden relative was an aylonis who is not subject to yibum at all, is found to be married in violation of the prohibition of "yevamah lashuk," and she must leave both husbands with all the penalties that this entails. We also learned that this ruling follows the view of Rabbi Akiva, who holds that even chayavei lavin produce a mamzer.
In the next Mishnah we will study another case in which the same penalties are imposed.