Gemara

(R. Chiya bar Ami - Beraisa): If a wife died during Kidushin, her husband is not considered an Onen (a mourner before burial of the Mes) and he need not become Tamei through engaging in her burial;

Similarly, if he died she is not an Onenes and need not become Teme'ah.

Mo'ed Katan 20b (Beraisa): One is Mis'avel (conducts Aveilus) upon losing any relative for whom the Torah commands a Kohen to become Tamei, i.e. a wife, parent, (paternal) brother or (virgin) sister, or child;

Chachamim added a brother or virgin sister from the mother, and a married sister from either parent.

Rishonim

Rif and Rosh (Mo'ed Katan 20b and 3:35): One is Mis'avel upon losing any relative for whom the Torah commands a Kohen to become Tamei, i.e. a wife, parent, (paternal) brother or (virgin) sister, or child. Chachamim added a brother or virgin sister from the mother, and a married sister from either parent.

Nimukei Yosef (DH v'Achoso): If one of these seven died on Yom Tov Sheni, Aveilus is mid'Oraisa, so it overrides Yom Tov Sheni, which is mid'Rabanan. One would not mourn on Yom Tov Sheni for relatives that Chachamim added. Aveilus mid'Rabanan of an individual does not override Yom Tov mid'Rabanan of the Rabim.

Rosh and Nimukei Yosef (ibid.): One is Mis'avel for his wife only if she was permitted to him. One is Mis'avel for other relatives whether or not they are Kesherim.

Rambam (Hilchos Evel 2:1): Mid'Oraisa one must be Mis'avel over a parent, child or paternal sibling. Mid'Rabanan one is Mis'avel over a wife if there was Nisu'in...

Radvaz: The Rambam's text did not include a wife with those for whom Aveilus is mid'Oraisa, rather, with those whom Chachamim added. They added relatives so that a person will be pained over their death.

Kesef Mishneh: The Rambam holds that we learn from Tum'as Kohanim over whom we are Mis'avel. Since mid'Oraisa a Kohen becomes Tamei for his Nesu'ah wife, also Aveilus should be mid'Oraisa! It seems that the Rambam calls it mid'Rabanan because Tum'ah for one's wife is not explicit in the Torah. We learn from "li'She'ero"; the Targum is 'li'Krivei'.

Lechem Mishneh: The Mizrachi says that a husband is Metamei for his wife mid'Rabanan. The verse is a mere Asmachta. It is not clear what is the source to say this. We learn from "Lo Yitama Ba'al b'Amav Lehechalo" that a husband become Tamei for a permitted wife, but not for a forbidden wife! It seems that the Rambam was forced to say that it is only mid'Rabanan because a husband is not Mis'avel with his wife over all her relatives. The Rambam rules that inheritance of a wife is only mid'Rabanan, and this is also expounded from li'Sh'ero. "Lo Yitama Ba'al b'Amav" need not refer to a husband (the Targum explains that it refers to a great man).

Rambam (ibid.): Mid'Rabanan one is Mis'avel over maternal siblings, and a wife over her husband if there was Nisu'in.

Kesef Mishneh: She is Mis'avel over her husband just like she becomes Tamei for him.

Rebuttal (Lechem Mishneh): The Rambam holds that the Torah does not require women to become Tamei for men! Rather, the Gemara says that a woman is Mis'avel over her father-in-law and mother-in-law for her husband's honor, and we infer that she is Mis'avel over him.

Gra (YD 374:4): We learn from the Mishnah (41a), which says that a widow may not become Mekudeshes until the Aveilus over her husband ends.

Rambam (2): Even a Kohen, who may not become Tamei for maternal siblings or even for a paternal sister who had Nisu'in, is Mis'avel over them. Aveilus over a paternal sister after Nisu'in is mid'Oraisa.

Questions (Ramban cited in Lechem Mishneh and Radvaz): The Beraisa says that Chachamim added a paternal sister after Nisu'in! We learn Aveilus from Tum'ah, and mid'Oraisa a Kohen may not become Tamei for her!

Poskim

Shulchan Aruch (YD 374:4): One is Mis'avel upon losing any relative for whom a Kohen becomes Tamei, i.e. a parent, child, (paternal) brother and paternal virgin sister, and wife. Chachamim added a maternal brother, a maternal sister whether or not she was married, and a married paternal sister, even though a Kohen is not Metamei for them.

Beis Yosef (DH Kol): When R. Yo'el's maternal married sister died he bathed, for he held that Chachamim added a married sister from the father and mother, i.e. a full sister. All the Poskim hold that Chachamim added a married sister from the father or mother.

Shulchan Aruch (ibid.): Just like a man is Mis'avel over his wife, a woman is Mis'avel over her husband. This is only if she was Nesu'ah and permitted to him. One is Mis'avel over a child or sibling even if the latter is Pasul.

Question (Beis Yosef DH veha'Mordechai): The Mordechai (Mo'ed Katan 910) brings from Maseches Semachos (4:4) that a Kohen is Mis'avel over a divorcee, and a Kohen Gadol over a widow. It is not clear why the Ramban and Rosh say otherwise. Perhaps they hold that the Gemara did not bring this Beraisa because it is not reliable.

Answer (Gra 5): We are Mis'avel over relatives for whom a Kohen becomes Tamei, and others that Chachamim added. A Kohen may not become Tamei for a Pasul wife, and Chachamim did not add this.