Does a Mitzvas Asei doche a lav by Yibum in the case of Mamzeirus?

Question

Hypothetical case: 2 half brothers with a common father. One brother is kosher and the other brother is a mamzer. The mamzer marries a mamzerres and they have no offspring. The mamzer dies and the mamzerres is now "zakuk leyibbum" bound by her brother in law and cannot marry any one else until he does yibbum or chalitza.

Does the kosher brother in law have to do chalitza as he is prohibited to marry a mamzeres?

Or maybe she can do yibbum as the asei (positive mitzva), "Yevama yavo aleha" overrides the lo taaseh (negative mitzva) of "lo yavo mamzer bikhal hashem" ?

The question would likewise apply if the kosher brother dies childless leaving his kosher wife to be zakuk (bound) to his brother who is a mamzer.

The other cases where this question would be applicable Would be in cases of Petzua daka (a man with crushed reproductive organ), Kerus shofcho (a man with a cut off reproductive organ), Almono Lecohen Gadol (women that had been betrothed and become a widow, to do yibbum to a high priest), Gerusha lecohen Hediot (divorcee to any Priest),

Answer 1

In short, Biblically the initial act of cohabitation which constitutes yibbum would be permitted, but subsequent acts would be forbidden. The rabbis forbade even the initial act; and instead instructed halitsa, lest the relationship continue.

Rambam writes this in Hilkhot Yibum V'halitsa (6:11-2):

הייתה היבמה אסורה על יבמה איסור לאו, או איסור עשה, או שהייתה שנייה--הרי זו חולצת, ולא מתייבמת

יב ומן הדין היה שיתייבמו, שהייבום מצות עשה, וכל מקום שאתה מוצא עשה ולא תעשה, יבוא עשה וידחה את לא תעשה; אבל חכמים גזרו, שלא יתייבמו חייבי לאוין ולא שנייות--גזירה שמא יבוא עליה ביאה שנייה, והרי ביאתה אסורה ואין שם מצוה: שאין מצות עשה, אלא ביאה ראשונה בלבד

If the yevama is prohibitted to her yavam with a negative commandment, or a positive commandment, or a rabbinic commandment, she gets halitsa; not yibum...

It would have been logical that thy should get yibbum, for yibbum is a positive commandment and every place that you find a positive commandment and a [contradictory] negative commandment, the positive commandment supersedes the negative commandment. But the rabbis decreed that those who are forbidden with a negative commandment, and those rabbinically forbidden should not get yibbum, as an enactment lest he cohabit with her a second time, for sexual relations are forbidden [at that point] and there is no mitsvah. For there is only a mitsvah with the first cohabitation.

This is based on Yevamot 20b. See also Shulhan Arukh (EH 174:1).

A mamzer would be a typical example of someone forbidden with a negative commandment.