['His sister' in our Mishnah is] his maternal sister, who married his paternal brother.

['His mother's sister' in our Mishnah is when] a man and his son married two sisters. (The living brother is the son of the sister who married his father; his brother, who died, had a different mother.)

Achos Ishto [occurs when] two brothers married two sisters.

(Mishnah): His maternal brother's wife [does not do Yibum].

Source #1: It says here Achvah ("Ki Yeshvu Achim Yachdav"), and it says there "Shneim Asar Avadecha Achim Anachnu" - just like there, it discusses paternal brothers, also here (the Mitzvah of Yibum) discusses paternal brothers.

Source #2 (R. Yonason): It says here Yeshivah ("Ki Yeshvu Achim Yachdav"), and it says there Yeshivah - "vi'Rishtah v'Yashavta Bah." Just like Yeshivah there has inheritance with it, also Yeshivah here (regarding Yibum) has inheritance with it (they are paternal brothers).

Source #3 (R. Avun bar Bisna citing R. Yonason of Beis Guvrin): "Yeshvu [Achim Yachdav]" refers to dwelling in one house. This excludes maternal brothers, for this one goes to his father's house, and this one goes to his father's house.

"Yachdav" excludes Eshes Achiv she'Lo Hayah b'Olamo

Question (R. Bun bar Chiyah citing R. Ba bar Mamal): Ailonis and Eshes Achiv she'Lo Hayah b'Olamo were in the Parshah. Why do you say that Tzaras Ailonis is permitted [to do Yibum], and Tzaras Eshes Achiv she'Lo Hayah b'Olamo is forbidden?

Answer (R. Bun bar Chiyah): There is a different reason to exclude an Ailonis [from Yibum or Chalitzah] - "Asher Teled" excludes this (an Ailonis), who cannot give birth;

Is it not enough that she does not do Yibum - do you want to forbid her Tzarah?!

However, Eshes Achiv she'Lo Hayah b'Olamo is Ervah, and Ervah exempts her Tzarah.

(Asi): Tzaras Ailonis is forbidden.

Question: [The coming] Mishnah argues with Asi! If any of these died, did Mi'un, was divorced, or was found to be an Ailonis, the Tzaros are permitted.

Answer (R. Bun bar Chiyah citing R. Ba bar Mamal): Asi holds like R. Meir. R. Meir says that whenever you cannot do Yibum with me, you cannot do Yibum with my Tzarah.

(Mishnah): His daughter-in-law.

Inference (R. Yosi): (Our Mishnah discusses cases that arise without transgression.) This shows that one may marry the wife of his brother's son [after she was widowed or divorced. MAR'EH HA'PANIM - do not say that it is forbidden, lest he die without children and the Mitzvah of Yibum will be Batel, e.g. if she is the only widow, and the only Yavam is her father. MEICHAL HA'MAYIM - we did not decree to make this a Sheniyah, i.e. Ervah mid'Rabanan.]