Gemara

(Mishnah): If a woman's sons converted with her, the sons do not do Yibum or Chalitzah to each other's wives. This is even if the first son was conceived before conversion and born after conversion, and the second was conceived and born after conversion. The same applies to a slave whose sons were freed with her.

(Gemara): The children of the female slave Yudan were freed. Rav Acha bar Yakov permitted them to marry each other's wives.

Objection (Rava): Rav Sheshes forbade this!

Rav Acha bar Yakov: He forbade, and I permit!

Version #1: If they are only paternal brothers (from different mothers), all agree that they are permitted. If they are only maternal brothers, all agree that they are forbidden. They argue about full brothers:

Rav Acha bar Yakov permits, since people consider them to be paternal brothers. They call them 'the sons of Ploni';

Rav Sheshes forbids, for people also call them 'the sons of Plonis (their mother).'

Version #2: Rav Acha bar Yakov permits in all cases, even when they are only maternal brothers, because a convert is like a newborn baby.

Question (Beraisa): If two twins converted or were freed, they do not do Chalitzah or Yibum to each other's wives. If one married the other's wife, he is exempt;

If they were conceived before conversion and born after conversion, they do not do Chalitzah or Yibum, but one is liable for the other's wife;

If they were conceived and born after conversion, they are like Yisraelim in all respects.

Suggestion: The Reisha teaches that one is not liable for the other's wife. This implies that he is not liable, but it is forbidden!

Answer: Really, it is permitted. Since the Seifa says that he is liable, for parallel structure the Reisha says that he is exempt.

(Rava): People say that a Mitzri (Nochri) has no father. Do not say that this is because they have Bi'ah wantonly, and we do not know who the father is, but if we would know, we would be concerned;

Rather, even when we know, we are not concerned. Twins come from the same drop of semen, just it split into two, and the Beraisa teaches that they do not do Chalitzah or Yibum!

The Torah considers the child to have no father because "Their flesh is flesh of donkeys, and their seed is like that of horses."

(Beraisa): If a boy was born after his mother converted, but was conceived beforehand, he is considered a convert. He is forbidden to maternal relatives, but not to paternal relatives;

If he married his maternal sister or his father's maternal sister, he must divorce her. If he married his paternal sister or his father's paternal sister, they may stay married. If he married his mother's maternal sister, he must divorce her.

R. Meir says, if he married his mother's paternal sister, he must divorce her;

R. Meir says that he must divorce any relative on the mother's side, but he may keep any relative on the father's side.

Chachamim say, he may keep her.

He is permitted to the wife of his (maternal) brother, and the wife of his father's brother, and all other Arayos are permitted, i.e. even his father's wife.

Question: The Beraisa permits his brother's wife (even if his brother married her after conversion)!

Answer: No, he married her before conversion (we decree due to when he married her after conversion).

(Beraisa): If he married a woman and her daughter, he has Nisu'in with one and divorces the other. L'Chatchilah, he should not do Nisu'in.

Objection: If he must divorce her, of course l'Chatchilah he may not do Nisu'in!

Answer: The last clause refers to the cases in which he may keep her. L'Chatchilah, he should not do Nisu'in.

Rishonim

Rif and Rosh (11:2): All permit a convert to marry the wife of a solely paternal brother, but not the wife of a solely maternal brother. Regarding the wife of a full brother, Rav Acha bar Yakov permits and Rav Sheshes forbids. Some say that Rav Acha bar Yakov permits even a maternal brother's wife, because a convert is like a newborn baby. The Gemara asked against Rav Acha and answered. Nevertheless, since the Gemara did not say that the Halachah follows him, we are stringent like Rav Sheshes.

Nimukei Yosef (Reish 32a): The Ritva rules like the Poskim who are lenient like Rav Acha. The Isur is mid'Rabanan. Also he was lenient in practice against Rav Sheshes' ruling. This shows that he was sure.

Rambam (Hilchos Isurei Bi'ah 14:13): If a convert was married to his mother or maternal sister before conversion and they converted, we separate them. If he was married to a different Ervah and they converted, we do not separate them. Mid'Rabanan, a convert is forbidden to his mother's relatives. He is permitted to paternal relatives, even if they are Vadai, e.g. twins who surely have the same father. Therefore, a convert may marry the wife of his paternal brother or uncle, his father's wife or his daughter-in-law, even if she married his relative after they converted. Similarly, he is permitted to his mother's paternal sister, his paternal sister or his daughter after she converted. He may not marry his maternal sister, his mother's maternal sister, or his maternal brother's wife after his brother converted. If she was married to his brother before he converted, she is permitted.

Rebuttal (Ra'avad): The Beraisa forbids maternal relatives and permits paternal relatives. Surely, both discuss the same case (before or after conversion)! Rav Sheshes establishes it to be before conversion, but after conversion they are forbidden.

Defense (Magid Mishneh): We first asked from a Beraisa that permits Eshes Ach. This connotes even a purely maternal sister. In the first version, even Rav Acha needed to establish it to discuss before conversion. The next question was from a different Beraisa. It is not difficult to say that it discusses after conversion.

Magid Mishneh: The Rambam permits even twins, who have the same father and mother. (The Ramban and Rashba disagree.) This shows that he is lenient like Rav Acha. He forbids only maternal relatives who are not paternal relatives, like the first version. He did not follow the second version, for we may not adopt all the leniencies. Alternatively, the Rambam rules like Rav Sheshes in the second version. He explains that in the second version Rav Acha permits even purely maternal relatives, and Rav Sheshes argues only in this case. My first explanation is primary. A Beraisa forbids his mother's paternal sister, but this was according to R. Eliezer. We hold like R. Akiva who permits the mother's maternal sister to Benei No'ach, so he does not decree about her paternal sister.

Kesef Mishneh: The Rambam permits all paternal relatives, even the father's wife. The Rif permits 'purely paternal relatives'; it seems that he agrees. They can establish the Beraisa (98b) that permits Eshes Av to be even like R. Akiva. Tosfos and the Rosh say that it must be like R. Eliezer.

Poskim

Shulchan Aruch (YD 269:2): If a convert was married to his mother or maternal sister before conversion and they converted, we separate them. If he was married to a different Ervah and they converted, we do not separate them.

Shulchan Aruch (3): Mid'Rabanan, a convert is forbidden to his mother's relatives. He is permitted to his father's relatives, even if they are Vadai, e.g. twins who surely have the same father. Therefore, a convert may marry the wife of his paternal brother or uncle, his father's wife or his daughter-in-law, even if she married his relative after they converted. Similarly, he is permitted to his mother's paternal sister, his paternal sister or his daughter after she converted.

Rema: Some forbid his father's wife.

Shach (4): According to Tosfos and the Rosh, we should also be stringent about his father's sister.

Gra (8): The Rema cites the Tur. Really, the Tur is stringent also like R. Eliezer, and forbids paternal sisters and aunts. He is lenient only about paternal relatives through marriage.

Shulchan Aruch (ibid.): He may not marry his maternal sister, his mother's maternal sister, or his maternal brother's wife after his brother converted. If she was married to his brother before he converted, she is permitted.