The mishnah on Yevamot 97b states:
A female convert whose sons converted with her, they do not perform halitzah (for each other’s wives), and they do not perform levirate marriage with them.
The Ritva, a medieval Spanish commentator, explains our mishnah by noting that a convert to Judaism is considered “like a newborn babe.” In other words, once a person converts, they are no longer considered related to their immediate family members. According to Jewish law, conversion acts as a reset not only for a convert’s religious life, but for their family connections, too.
This is important because, as we have learned, yibbum or halitzah is performed by the brother of a childless man who has died. But if one or both of the brothers converted to Judaism, they are no longer considered to be siblings at all and therefore are exempted from halitzah and yibbum.
Immediately, the Gemara brings a story to illustrate a rabbinic dispute about the application of our mishnah.
The sons of Yudan the maidservant were freed. Rav Aha bar Ya’akov allowed them to marry each other’s wives after divorce. Rava said to him: Didn’t Rav Sheshet prohibit it? Rav Aha bar Ya’akov said to him: He prohibited it, and I permit it.
In this story, the sons of a maidservant were freed from enslavement, and the Gemara seems to presume that they then converted to Judaism. Rav Aha bar Ya’akov, relying on the principle that brothers who converted no longer have the legal status of brothers, allowed them to marry each other’s divorcees, which would normally be forbidden. Rava, however, doesn’t like that ruling. He reminds Rav Aha bar Yaakov that Rav Sheshet prohibited such a marriage. (We will learn why he did so shortly.) Rav Aha bar Yaakov answers, in effect, he’s wrong and I’m right!
Rav Aha bar Yaakov seems to be on firm ground in applying the principle of our mishnah that familial relationships are invalidated if two brothers convert to Judaism. So why then does Rav Sheshet rule otherwise?
After a short conversation about whether this ruling is the same for paternal brothers as for maternal brothers, Rav Sheshet concludes:
They are also called the sons of so-and-so (their mother).
Rashi explains that Rav Sheshet is concerned that while knowledgeable people are aware that converts are no longer considered brothers, the public might not be, since the brothers are still known as the sons of their mother. Consequently, they may think that if these two brothers can marry each other’s divorcees, then everyone can.
Of course, we already know from earlier in Yevamot that it is typically forbidden to marry one’s sister-in-law; the sole exception is in order to perform yibbum for a dead brother. Rav Sheshet here is putting a “fence around the Torah” — adding an extra prohibition to keep people from violating the halakhah.
This mishnah and the story of Yudan and her sons teach us not only about the law concerning converts, but about the makeup of Jewish communities in the time of the Talmud. While the rabbis themselves are aware of the finer points of the law (since they wrote it), regular people who did not spend their days studying the oral tradition might not be. Not knowing that this situation is an exception, rather than the rule, they might inadvertently violate the law if they emulate the brothers’ example.
Rav Sheshet is no doubt aware of this dynamic, and so he forbids such a union. Rav Aha bar Yaakov differs, relying on the letter of the law. Rav Aha bar Yaakov has the law on his side. Rav Sheshet has human nature on his. Who wins out?
The Shulchan Aruch, the definitive medieval law code, comes down on the side of Rav Sheshet. (Yoreh De’ah 269:1) This ruling reflects the understanding that human desire to emulate the behavior of others might be more influential than the law itself. Therefore, a fence around the law is necessary to keep the people from sinning.
Read all of Yevamot 97 on Sefaria.