Gemara

(Beraisa - R. Meir and R. Yehudah): A Kohen makes himself Tamei to bury his sister ha'Arusah (Mekudeshes);

R. Yosi and R. Shimon say, he does not.

If she was raped or enticed, all agree he does not Metamei himself for her.

R. Shimon says, if she is a Mukas Etz he does not Metamei himself, for a Kohen is Metamei himself for a sister only if she was permitted to a Kohen Gadol.

All say that he is Metamei himself for a Bogeres.

Question: What is the reason of R. Meir and R. Yehudah?

Answer: "Ha'Besulah" excludes one who was raped or enticed.

Suggestion: We should exclude even a Mukas Etz!

Rejection: "Asher Lo Haysah l'Ish" connotes one whose Havayah (loss of virginity) was through a man. This excludes a Mukas Etz.

"That is close" includes an Arusah. "To him" includes a Bogeres (even though some of her Besulim fell out).

Question: Why must a verse include Bogeres? R. Meir holds that "Besulah" includes one who has even some Besulim!

Answer: One might have thought that a Gezeirah Shavah "Besulah-Besulah" from there (Rashi - a virgin who was raped; Tosfos - from a girl fit for a Kohen Gadol) teaches that she must be a Na'arah;

The verse teaches that we do not.

Question: How do R. Yosi and R. Shimon learn?

Answer: "Ha'Besulah" excludes one who was raped or enticed and a Mukas Etz;

"Who was not (married)" excludes an Arusah. "that is close" includes an Arusah who was divorced;

"To him" includes a Bogeres.

Question: How can "That is close" include an Arusah who was divorced? R. Shimon says that he is Metamei himself for a sister only if she was permitted to a Kohen Gadol!

Answer: This case is different. The Torah said "close" to include it.

Question: We should say that "close" includes also a Mukas Etz!

Answer: It comes to include one case, not two.

Question: Why do we say that it includes an Arusah who was divorced, and not a Mukas Etz?

Answer: A physical act happened to the latter, but not to the former.

Question: R. Yosi did not say the law of Mukas Etz with his partner (R. Shimon), so he must hold like R. Meir. How does he expound?

Answer: "Who was not to a man" includes a Mukas Etz.

Question: He already used that verse (to exclude an Arusah)!

Answer: "Was not" excludes an Arusah. "To a man" includes a Mukas Etz.

"To him" includes a Bogeres.

Question: R. Shimon said (regarding a Kohen Gadol) that "Besulah" connotes having all the Besulim! (Tosfos Yeshanim - he should learn from here that Besulah connotes even some Besulim, for a Bogeres is included!)

Answer: He learned that from our verse;

Since the Torah needed to write "To him" to include a Bogeres, we infer that "Besulah" connotes all the Besulim.

Rishonim

The Rif and Rosh (Hilchos Tum'ah (after Menachos) 1b and Siman 5) bring our Gemara. The Halachah does not follow R. Shimon regarding Mukas Etz, for the other three Tana'im oppose him. The Halachah follows R. Yosi and R. Shimon regarding an Arusah against R. Meir and R. Yehudah, because the Halachah follows R. Yosi against either opponent.

Question (Nimukei Yosef DH Garsinan): Above, the Rif brought "Ha'Kerovah" to exclude an Arusah and a divorcee. This is unlike R. Meir, who permits becoming for an Arusah even before divorce, and unlike R. Yosi, who permits for an Arusah who was divorced!

Ma'adanei Yom Tov (20): That Drashah refers to becoming Tamei for one's wife. This is different than the law of Tum'ah for one's sister! The Nimukei Yosef's question was based on his errant text. It says "ha'Kerovah" regarding a sister. Regarding one's wife it says "Li'Sh'ero ha'Karov Elav"

Rambam (Hilchos Evel 2:10): A Kohen is not Metamei himself for his sister who had Nisu'in, even if she married a Kohen. We learn from "Ha'Besulah ha'Kerovah Asher Lo Haysah l'Ish". "Ha'Besulah" excludes one who was raped or enticed. We do not exclude a Bogeres or Mukas Etz, for "Asher Lo Haysah l'Ish" connotes one whose Havayah was through a man. "Asher Lo Haysah l'Ish" teaches that he does not Metamei himself for his sister ha'Arusah, even if she is married to a Kohen.

Lechem Mishneh: The Gemara includes Bogeres from "Elav"! Tosfos asked why we do not learn also Bogeres from Asher Lo Haysah l'Ish. He answered that if not for "Elav", we would have said that it includes only Bogeres, but not Mukas Etz. Now that it says "Elav", we learn both from Asher Lo Haysah l'Ish.

Rambam (11): If she was divorced from Eirusin he becomes Tamei for her. "Ha'Kerovah" includes this.

Radvaz: If she had Nisu'in he can never become Tamei for her even if there was no Bi'ah and she was divorced.

Poskim

Shulchan Aruch (YD 373:4): A Kohen is not Metamei himself for his sister ha'Arusah, even if she is Arusah to a Kohen.

Mishneh l'Melech (Evel 2:7): He is Metamei for her if she was Mekudeshes mid'Rabanan. A Kohen can be Metamei for his mid'Rabanan wife only because she is like a Mes Mitzvah (Yevamos 89b). This does not apply here, so Kidushin mid'Rabanan does not uproot the Mitzvah mid'Oraisa of Tum'as Kerovim.

Rebuttal (Pischei Teshuvah 3): The Gemara there asked whether Chachamim can tell people to transgress Torah law through an action. It explained that her husband can become Tamei because she is like a Mes Mitzvah. We know that Chachamim can tell people to passive refrain from Mitzvos!

Shulchan Aruch (ibid.): He cannot Metamei for her if she was raped or enticed. He becomes Tamei for a sister who was divorced from Eirusin, or a Mukas Etz.

Gilyon Maharsha (DH Metamei): Tosfos (Kesuvos 49a DH v'Eima) says that a Nesu'ah is always considered a Be'ulah.