Gemara

23b (Mishnah): If a man was Mekadesh one of two sisters and does not know which, he divorces both.

If he died leaving one brother, he does Chalitzah to both;

If he left two brothers, one does Chalitzah and the other Yibum. If both did Yibum, they may keep their wives.

24a (Shila - Beraisa): This is even if they are Kohanim.

This is because the Isur Chalutzah l'Kohen is only mid'Rabanan. Chachamim did not forbid a Safek.

(Mishnah): If Shimon died and Reuven did Chalitzah to Leah, he is forbidden to her relatives, and she is forbidden to his;

He may not marry her mother, grandmothers, daughter, or granddaughters, and not her sister in her lifetime;

The brothers are permitted to her relatives. She is forbidden to Reuven's father, his father's father, his son, his son's son, his brother, and his brother's son;

A man is permitted to the relative of the Tzarah of his Chalutzah, but forbidden to the Tzarah of the relative of his Chalutzah.

(Gemara) Question: Did Chachamim decree Sheniyos regarding a Chalutzah?

Did they forbid Shniyos only of Ervah mid'Oraisa, but not of a Chalutzah?

Or, did they not distinguish (and decreed even regarding a Chalutzah)?

Answer #1 (Mishnah): Reuven is forbidden to Leah's mother, grandmothers...

It does not list her mother's mother's mother (because we did not decree)!

Objection: Perhaps the great grandmother is really forbidden. The Tana omitted it because the Seifa teaches that the brothers are permitted to Leah's relatives. Had the Reisha included the great grandmother, one might have thought that the Seifa permits the brothers only to her, but not to Leah's mother and grandmothers.

Question: It could have included the Isur of the great grandmother, and said that the brothers are permitted to all of them!

This is left difficult.

Answer #2 (R. Chiya's Beraisa): Four are forbidden mid'Oraisa, and four mid'Rabanan. His father, son, brother and brother's son are mid'Oraisa. His father's father, mother's father, the son of his son, and the son of his daughter are mid'Rabanan.

Suggestion: The father of his father is due to the Choletz. She is like his son's daughter-in-law (a Shniyah)!

Rejection: No, it is due to the deceased (she truly is his son's daughter-in-law).

Suggestion: The father of his mother is due to the Choletz. She is like his daughter's daughter-in-law (a Shniyah)!

Rejection: No, she is like his daughter's daughter-in-law due to the deceased.

Suggestion: The son of his son is due to the Choletz. She is like his father's father's wife (a Shniyah)!

Rejection: No, it is due to the deceased, she is like the wife of the brother of his father's father.

Question: But Ameimar permits the wife of the brother of one's father's father!

Answer: Ameimar must say that it is due to the Choletz, i.e. we decreed Sheniyos regarding a Chalutzah.

Suggestion: The son of his daughter is due to the Choletz. She is like the wife of his mother's father (a Shniyah)!

Rejection: No, it is due to the deceased. She is the wife of the brother of his mother's father.

Objection: The wife of the brother of one's mother's father is permitted!

Conclusion: Rather, we must say that it is due to the Choletz. We did decree Sheniyos regarding a Chalutzah.

Rishonim

Rif and Rosh (4:19): The conclusion is that Chachamim decreed Shniyos regarding a Chalutzah.

Nimukei Yosef (DH Masnisin): Chachamim decreed to forbid to the Yavam all relatives of his Chalutzah that would be forbidden had she married him.

Rambam (Hilchos Yibum 1:13): Just like the Chalutzah is forbidden to the Yavam, also her relatives such as her mother and daughter. Similarly, she is forbidden to his son and brother. Even her (relatives forbidden to her husband due to) Shniyos, such as her daughter's daughter's daughter are forbidden (to the Yavam), and she is forbidden to (Shniyos, e.g.) his son's son's son. The rule is, she is like his divorcee.

Rambam (6:13): If Reuven was Mekadesh Leah b'Safek, and her sister Rachel fell to him to Yibum, he must do Chalitzah, she is Safek Achos Ishto. He must divorce his wife, for she is Safek Krovas Chalutzaso, which like a Shniyah.

Poskim

Shulchan Aruch (EH 162:3): One who does Chalitzah is forbidden mid'Rabanan to her relatives, even Shniyos. Similarly, she is forbidden to his relatives. The brothers are permitted to all her relatives.

Rema: Even if he did Chalitzah due to Safek, he is forbidden to all her relatives.

Source (Mordechai 21): We do not allow a Safek Chalutzah to a Kohen (36b) because Isurei Kehunah are more stringent. Even Re'ach ha'Get disqualifies to Kehunah. The decree to forbid a Chalutzah to Kehunah is due to a divorcee. R. Baruch did not want to permit a Safek Achos Chalutzaso. Even though it is mid'Rabanan, sometimes Chachamim were stringent about a Sefek-Sefeka mid'Oraisa, which is like a Safek mid'Rabanan. E.g. a Tumtum may not do Yibum, for perhaps he is male, and if he is male perhaps he is a Seris, who may not do Yibum. One may not sell a Safek Tereifah to a Nochri, lest he resell it to a Yisrael. Any Safek Tum'ah in Reshus ha'Yachid is Tamei mid'Rabanan, no matter how many Sefekos are involved. However, there we should have followed Chezkas Taharah, but the Torah said that it is Tamei, so we do not distinguish how many Sefekos there are. However, above (24a) we permit a Safek Chalutzah! It seems that this is after Chalitzah was already done, but when a Kohen married a Safek Yevamah l'Shuk, we do not do Chalitzah lest this disqualify the children.

Gra (3): Re'ach ha'Get does not show that we are more stringent about Isurei Kehunah, for it also forbids relatives.

Source #2 (Nimukei Yosef 5a DH Masnisin and Gra 3): If we do not know which of two sisters fell to Yibum, if there is one Yavam, he does Chalitzah to both. If there are two Yevamim, one does Chalitzah and the other Yibum. If both did Yibum, they may keep their wives. When there is one Yavam, we do not say that if he did Chalitzah to one and Yibum to the other that he may keep her! Even though Achos Chalutzaso is mid'Rabanan and it is a Safek (perhaps she is really the Yevamah), we are stringent lest she be confused with Achos Gerushaso, which is Kares.

Question (Be'er Heitev 3): In various places we permit Safek Chalutzah!

Shulchan Aruch (173:3): If Reuven was Mekadesh Leah b'Safek, and her sister Rachel fell to him to Yibum, he must do Chalitzah, for she is Safek Achos Ishto. He must divorce his wife, for she is Safek Krovas Chalutzaso.

Bach (DH ha'Yevamah and DH Efshar): The Rambam says so. It is not clear what his source is. The Rif and Rosh forbid only Safek Achos Zekukaso. R. Baruch did not permit l'Ma'aseh, but it seems that he has no source to forbid. The Gemara (24a) said that if both did Yibum, they may keep their wives even if they are Kohanim, because the Isur Chalutzah l'Kohen is only mid'Rabanan, and Chachamim did not forbid a Safek. Likewise, Safek Achos Chalutzaso is forbidden l'Chatchilah, but permitted b'Di'eved.

Rebuttal (Beis Shmuel 3): This is unlike the Nimukei Yosef. The Bach himself brings the Nimukei Yosef later (Siman 176 DH u'Mah she'Chosav Mes)!