The disagreement between Rav Aḥa bar Ya’akov and Rav Sheshet TB Yevamot 98

Generally speaking when somebody converts and joins the Jewish people, Jewish law considers his/her legal status as “a convert who just converted is like that of a child just born, and all his previous family relationships are disregarded, whether from his father’s side or from his mother’s side (גֵּר שֶׁנִּתְגַּיֵּיר כְּקָטָן שֶׁנּוֹלַד דָּמֵי)” (Sefaria.org translation) Daf TB Yevamot 97b records a disagreement between Rav Aḥa bar Ya’akov and Rav Sheshet.

“The sons of Yudan the maidservant were freed. Rav Aḥa bar Ya’akov allowed them to marry each other’s wives after divorce. Rava said to him: Didn’t Rav Sheshet prohibit marriage in that case? Rav Aḥa bar Ya’akov said to him: He prohibited it and I permit it. I disagree with his ruling.

“The Gemara explains: If the two freed slaves or converts are half brothers from their father’s side and not from their mother’s side, everyone agrees that the marriage is permitted, as even a gentile and certainly a convert are considered unrelated to their father’s family. If they are half brothers from their mother and not from their father, everyone agrees that it is prohibited.

“When they disagree, it is a case where they are brothers both from their father and from their mother. The one who permits the marriage claims that we trace them after their father. Their paternal lineage is followed, since they are called the sons of so-and-so, their father. Since they are recognized by their paternal lineage, it is well known that they are considered unrelated, and there is no concern that people will infer that a man may marry his sister-in-law. And Rav Sheshet maintains that they are also called the sons of so-and-so, their mother. Therefore, this concern does exist, as it is not common knowledge that a convert is considered reborn and unrelated to his mother’s family.

“And some say a different version of this dispute: Rav Aḥa bar Ya’akov disagrees with Rav Sheshet, even with regard to maternal half brothers. And what is the reasoning behind this opinion? The legal status of a convert who just converted is like that of a child just born, and all his previous family relationships are disregarded, whether from his father’s side or from his mother’s side.” (Sefaria.org translation)

Today’s daf TB Yevamot 98 summons a proof bolstering Rav Aḥa bar Ya’akov position.

“The Gemara resumes discussion of the dispute between Rav Aḥa bar Ya’akov and Rav Sheshet. Come and hear another proof: A convert whose birth was in sanctity but whose conception was not in sanctity has maternal kinship, i.e., his relationship to his mother’s relatives is recognized. However, he does not have paternal kinship. How so? If he married his maternal half sister, who was born before him and converted, he must divorce her. Although by Torah law they are considered unrelated, the Sages rendered it prohibited for them to marry, lest he marry a maternal half sister who was born after him and is forbidden to him. If she is his paternal half sister, he may maintain her as his wife. If he married his father’s maternal half sister, he must divorce her.

“If she is his father’s paternal half sister, he may maintain her as his wife. If she is his mother’s maternal half sister, he must divorce her. If she is his mother’s paternal half sister, Rabbi Meir says he must divorce her, and the Rabbis say he may maintain her. This is as Rabbi Meir would say: Any relative forbidden due to kinship with the mother, whether the woman is his paternal relative, e.g., his father’s maternal half sister, or his maternal relative, he must divorce her. However, if she is forbidden due to the father, he may maintain her.

“And he is permitted to marry his maternal brother’s wife and his father’s brother’s wife, and all other forbidden relatives are also permitted to him. The expression: And all other relatives are also permitted to him, is added to include the father’s wife.” (Sefaria.org translation)

Rashi ד"ה נָשָׂא אֲחוֹתוֹ מִן הָאֵם explains the reason thusly. A baby is conceived when his mother was still a Gentile She converted and became Jewish before he was born. This baby is of course Jewish. For reasons I don’t really understand, his status can be described as Jewish-. His mother has a previous daughter from another marriage who also converts. He can’t marry this half sister even though halakhicly they are not related. If his mother now has a Jewish baby girl both conceived and born Jewish, her brother with the status Jewish- certainly can’t marry her. The rabbis made an enactment prohibiting him from marrying the half-sister because he can’t marry his full sister.

Another Gemara elsewhere gives a different explanation why he is forbidden to marry his half-sister. According to the Torah Gentiles are obligated to observe the seven commandments of Noah. One of the seven prohibitions is illicit relations. Once siblings convert according to Jewish law they are not “related” to each other. The rabbis were afraid that people would convert only for the purpose of marrying their sister. Consequently, they forbade this marriage even though technically according to Jewish law they are not related.

Tosefot ד"ה נָשָׂא אֲחוֹתוֹ מִן הָאֵם provides a third reason. Even though the boy in our story is Jewish-, he looks Jewish because nobody knows his circumstance that he was conceived while his mother was still a Gentile and his half-sister converted on her own. Consequently Jews will think that one may marry one’s own sister when they see him marrying his half-sister without knowing the facts of the case. Consequently, the rabbis forbade this marriage.