Gemara

3a (Beraisa - R. Yishmael): "He will become Tamei for her (his deceased sister)" is optional;

R. Akiva says, it is an obligation.

Zevachim 100a (Beraisa - R. Akiva): (A Nazir may not Metamei himself to bury a) "Nefesh" - this refers to a relative. "Mes" refers to a stranger. He may not be Mitamei "l'Aviv" (for his father), but he is Mitamei for a Mes Mitzvah.

"L'Imo" - a Kohen Nazir may not Metamei himself for his mother, but he is Metamei himself for a Mes Mitzvah.

Yevamos 29b (Beraisa): If an Arusah (an engaged wife) died, her husband is not an Onen (forbidden to Kodshim) due to her. He does not become Tamei for her. If he died, she is not an Onenes due to him, and does not become Tamei for him.

Sukah 25b: One engaged in a Mitzvah is exempt from other Mitzvos.

(Beraisa - R. Yitzchak): "There were Tamei people (who could not offer Korban Pesach)" - they engaged in a Mes Mitzvah. Their seventh day was Erev Pesach.

Rishonim

Rambam (Hilchos Evel 2:6): "Lah Yitama" is an Aseh for a male Kohen to be Mitamei for his relatives. If he does not want to, we force him. A Kohenes has no Isur to be Mitamei, therefore she need not be Mitamei for relatives. It is optional.

Rebuttal #1 (Ra'avad): An Arusah need not become Temei'ah for her husband. This implies that a Nesu'ah must become Temei'ah for him.

Rebuttal #2 (Ramach): If a Kohen is forbidden to become Tamei, yet he is commanded to be Mitamei for relatives, all the more so a Bas Kohen, who may become Tamei, is commanded to be Mitamei for relatives!

Note: This Kal va'Chomer applies also to Yisre'elim.

Toras ha'Adam (Inyan Kohanim 36, cited partially in Beis Yosef YD 373 DH veha'Ramban): Perhaps the Rambam expounds that "Yetamei" applies only to one for whom "Lo Yitama" applies. Alternatively, "Benei Aharon" excludes Benos Aharon from the entire Parshah.

Sha'agas Aryeh (ha'Chadashos 8): Both reasons exclude also Yisre'elim.

Rebuttal (Imrei Baruch on Turei Even Rosh Hashanah 16a, DH Od): When explaining the latter reason, the Ramban says 'a Kal va'Chomer exempts Bnos Yisrael. Kohanim have extra Mitzvos, yet Kohanos are exempt. All the more so, (even though Bnei Yisrael are obligated,) Benos Yisrael are exempt.

Sha'agas Aryeh (ibid.): In Yevamos, Rashi explained that an Arus Kohen may not be Mitamei for his Arusah, and an Arusah Yisraelis or Kohenes need not be Metamei for him. Why didn't he explain also that an Arus Yisrael or Kohen need not be Metamei for her? Rashi's second Perush forbids her to become Tamei for her Arus during the festival, when even Yisre'elim are commanded about Tum'ah - "uv'Nivlasam Lo Siga'u." This Isur is so that Yisrael can bring Shalmei Simchah. (Only a seven-day Tum'ah would Mevatel Olas Re'iyah and Chagigah.) The Rambam (Chagigah 1:1) obligates women in Simchah, like R. Zeira. (The Kesef Mishneh erred about this.) Therefore, women are commanded about Tum'ah during the festival, so the Rambam can explain that an Arusah may not become Tamei for her Arus during the festival. The Ra'avad rules like Abaye, who exempts women from Simchah, so they are not commanded about Tum'ah during the festival. This forced him to explain that she is not commanded to be Mitamei for her Arus.

Tosfos (Sukah 25b DH Mishum): In Zevachim, we say that one going to slaughter his Pesach is Mitamei (for a relative only before midday, but) for a Mes Mitzvah even after midday. Here (in Sukah), we permit for a Mes Mitzvah only before the Chiyuv came! We do not discuss a proper Mes Mitzvah, rather, a relative.

Turei Even (Rosh Hashanah 16a DH Chayav): Also Rashi (DH Teme'ei) explains that the Mes Mitzvah (a week before Pesach) that was Docheh Pesach was Tum'as Kerovim. A real Mes Mitzvah is Docheh everything, even Kedushas Kohen Gadol and Nazir. We could not try to learn from it to exempt one engaged in a Mitzvah from other Mitzvos! This shows that even Yisre'elim are commanded (the only Kohanim were Aharon and his sons)!

Minchas Chinuch (264 [23] (or p. 76a) DH v'Ayen): All of Yisrael needed to be Tahor to offer Pesach, so others could not be Mitamei. Therefore, any Mes was like a Mes Mitzvah! However, presumably some Yisre'elim were already Tamei, and they could have buried the Mes.

Note: Any Yisrael who had not become Tamei that day would have been able to become Tahor in time for Pesach! Eichah Rabah (Petichah 33, cited in Tosfos Bava Basra 121a) holds that in the Midbar, men died only on Tish'ah b'Av (with limited exceptions, e.g. Korach's rebellion). If the same applies to women, there is no reason to assume that other Yisre'elim were Temei'im!

Rosh (Hilchos Tum'ah 4): Rashi (Kesuvos 53a DH v'Lo Mitam'es) explains that 'an Arusah is not Metamei' does not apply to a Kohenes, for she is not commanded about Tum'ah. Rather, whether she is a Kohenes or Yisraelis, she is not commanded to be Mitamei for her Arus, but it is a Mitzvah to be Metamei for other relatives. The Rambam exempts.

Rashi (Yevamos 29b v'Lo Mitam'ah): I heard that (even though a woman must be Metamei for other relatives,) an Arusah need not be Metamei for her Arus. I say that an Arusah may not become Tamei for her Arus during the festival, when (even) Yisre'elim are commanded about Tum'ah - "uv'Nivlasam Lo Siga'u"

Rashba (1:27): It is optional for a Yisrael to be Mitamei for his relatives.

Poskim

Shulchan Aruch (YD 373:3): All relatives for whom a Kohen may become Tamei, it is a Mitzvah to be Mitamei for them. If he does not want to, we force him. The same applies to men and women.

Shach (3): A Kohenes is commanded to be Mitamei.

Rebuttal #1 (Sha'agas Aryeh ha'Chadashos 8 DH Nachzor): Rashi (Bava Metzi'a 18 DH v'Lo) says that the Mitzvah is even for Yisre'elim. All who argue with the Rambam mention 'women', i.e. even Bnos Yisrael. There is no reason to say that a Yisraelis is commanded, but a Yisrael is not.

Rebuttal #2: A Nazir may not be Mitamei "l'Aviv", but he is Mitamei for a Mes Mitzvah (Zevachim 100a). "L'Imo" teaches similarly about a Kohen Nazir. If there were no Mitzvah for a Yisrael, we could not learn about a Mes Mitzvah from "l'Aviv". We would need the verse to teach about a Kohen, that the Aseh of Tum'as Krovim does not override Nezirus!

Defense: If the Aseh of Tum'as Krovim applies only to Kohanim, it is not Shavah b'Chol (does not apply to everyone), so it does not override the Aseh and Lav of Nazir, even though Yeshnam b'She'elah (one can permit his vow)!

Retraction (Turei Even, ibid.): A verse teaches that a Metzora shaves even if he is a Kohen, and another permits even if he is a Nazir. We could not learn from one from the other, because it is not clear whether Yeshno b'She'elah or Eino Shavah b'Chol is a greater weakness. If the latter is stronger, such an Aseh would override the Aseh and Lav of Nazir, which are Yeshnam She'elah. Therefore, "l'Aviv" would be needed to teach that it is not!