Yevamot 22B

מיזקק נמי זקיק.

the husband’s brother also causes a levirate bond with his yevama even if he is a mamzer.

ואחיו הוא לכל דבר. למאי הלכתא? ליורשו וליטמא לו.

§ The Mishnah stated that he is his brother in all respects. The Talmud asks: With regard to what halakha was this said? The Talmud answers: To inherit from him, and to become ritually impure for him if he is a priest, as a brother is one of the sex for whom a priest is permitted to become ritually impure by attending his funeral.

פשיטא, אחיו הוא! סלקא דעתך אמינא: הואיל וכתיב: ״כי אם לשארו הקרוב אליו״, ואמר מר: ״שארו״⁠ — זו אשתו, וכתיב: ״לא יטמא בעל בעמיו להחלו״, יש בעל שמטמא ויש בעל שאין מטמא, הא כיצד? מטמא הוא לאשתו כשרה, ואין מטמא לאשתו פסולה,

The Talmud asks: Isn’t this obvious, as he is his brother? The Talmud answers: It might enter your mind to say that a priest does not become impure for his brother of flawed lineage, since it is written with regard to priests: “None should defile himself for the dead among his people except for his kin, that is near to him” (Leviticus 21:1–2), and the Master said “his kin”; this is his wife, and a priest may defile himself for his wife. But it is written: “He should not defile himself, being a husband among his people, to profane himself” (Leviticus 21:4), implying that he may not defile himself for his wife. This apparent contradiction is resolved as follows: There is a husband who does become impure for his wife, and there is a husband who does not become impure. How so? He becomes impure for his wife if she was fit and was permitted to a priest, but he does not become impure for his wife if she was unfit to marry a priest.