Yevamos 97b: If twin boys were in their mother’s womb and she converted to Judaism, and the twins grew up and married wives, and one died, the other need not do chalitzah or yibum, but he is forbidden by the Torah to take his brother’s wife.
The Nimukei Yosef (Yevamos 3b in the Rif’s page numbering) implies that they are only forbidden to take each other’s wives in the case of twins, but if a woman converted during pregnancy, had a baby, and then became pregnant again as a Jew and had a second baby, they would be allowed to take each other’s wives.
The Shach (Yoreh Deah 269:6) rules in accordance with the Nimukei Yosef.
יבמות צז ע”ב: שני אחים תאומים גרים, וכן משוחררים ־ לא חולצין ולא מייבמין, ואין חייבין משום אשת אחֹ. היתה הורתן שלא בקדושה ולידתן בקדושה ־ לא חולצין ולא מייבמין, אבל חייבין משום אשת אחֹ.
נימוקי יוסף ג ע”ב בדפי הרי”ף: אם נשא אשת אחיו או גר או עבד אינו חייב משום אשת אח אפילו היו לו בנים לאחיו ממנה כיון שהיתה הורתן ולידתן שלא בקדושה אבל היכא דהוו תאומים והיתה לידתן בקדושה חייבין משום אחותו ואשת אח ומשום דדמו לאחין מן האם ולא מן האב לכך אין מיבמין.
The Torah tells us, “And the children of Shimon were Yemuel, Yamin, Ohad, Yachin and Tzochar, and Shaul the son of the Canaanite woman.” (Bereishis 46:10) Rashi explains that “the Canaanite woman” refers to Dinah, who was taken captive by the Canaanite prince Sh’chem.
The commentators all ask: How could Shimon marry his full sister Dinah? True, the Torah had not been given yet, but even a non-Jew is forbidden to marry his sister, as long as they share a mother!
Rabbi Avrohom Tzvi Kamai zt”l hy”d, the last rav of Mir, answered as follows. The Torah describes Dinah’s birth: “And after that, she [Leah] gave birth to a daughter and named her Dinah.” (Bereishis 30:21)
The Gemara comments: After what? Rav said: After Leah judged herself and said: Twelve tribes will be born to Yaakov. Six have already come from me, and four from the maidservants – that makes ten. If this one is a boy, my sister Rochel will not even be equal to the maidservants. Immediately, the fetus became a daughter. (Berachos 60a)
The Targum Yonasan tells the story slightly differently. It’s not that the fetus was transformed from a son into a daughter, but rather Yosef was originally in Leah’s womb, while Dinah was in Rochel’s womb. The babies were miraculously switched, and Leah gave birth to Dinah.
According to this, Shimon’s conception was from Leah, while Dinah’s conception was from Rochel. At conception, they did not share a mother, only a father, and a non-Jew is allowed to marry his paternal sister. True, at birth they did share a mother. But we see from the Nimukei Yosef and the Shach that birth from the same mother is not enough to make people siblings. They must also have been conceived by the same mother.
Source: Mishulchan R’ Eliyahu Boruch, Vayigash
[The Nimukei Yosef does not explain why twins should be different. In the case where one fetus underwent geirus inside his mother, and thus is a ger, and the second baby was conceived after his mother’s geirus and is thus not a ger, they are permitted (on a Torah level) to take each other’s wives. If so, twins who underwent geirus in their mother’s womb should be the same. Since they were conceived as non-Jews, they are unrelated to each other.]
Read and Remember 