The Mishna lists fifteen women that exempt their co-wives and the co-wives of their co-wives from chalitzah and from yibum. One of them is his wife’s sister.
Tosfos asks: Why didn’t the Mishna include a case where the yevamah was a niddah at the time that she fell to yibum? She is forbidden to him at that time and he would be subject to the penalty of kares; wouldn’t there be a prohibition against performing a yibum with her even after she purifies herself?
Tosfos compares this case to the case listed in the Mishna; his wife’s sister. He cannot perform a yibum with her even if his wife would die (one is permitted to marry his wife’s sister after his wife dies). The explanation is: We consider the situation at the time when she falls to yibum; since at that time she was forbidden, she remains prohibited forever. Tosfos’ question is: Why don’t we apply the same logic to a woman who was a niddah at the time that she fell to yibum and just as she is forbidden at that time, she should remain prohibited even after she purifies herself?
Tosfos cites a Gemora in Pesachim (72b) which seems to indicate that a yavam who performs a yibum with a niddah had accomplished a mitzvah, and he has acquired her despite the fact that a transgression was committed.
(Tosfos answers this question by creating a distinction between his wife’s sister, who is only forbidden to him and a niddah, who is forbidden to everyone.)
The Acharonim ask: How can it be that the yavam has fulfilled the mitzvah of yibum by performing a yibum with a niddah; shouldn’t this be considered a mitzvah habaah b’aveirah, a mitzvah that is a result of an aveira that is performed? The positive commandment of yibum cannot override the negative transgression of engaging in relations with a niddah; this transgression is subject to the penalty of kares and cannot be overridden by a positive commandment. There are many Rishonim who maintain that one who performs a mitzvah and simultaneously commits an aveira does not fulfill the mitzvah at all.
Reb Elchonon Wasserman (Kovetz Heoros 11:1) answers: One does not fulfill the mitzvah by a case of mitzvah habaah b’aveirah when the mitzvah is merely an action, such as shaking a lulav or blowing a shofar; he cannot be credited with a mitzvah when that action was an aveira, as well. However, when the mitzvah is more than an action, but rather what the action accomplished, this principle is not applicable. If one circumcises a nine-day old baby on Shabbos, the milah is valid and he has fulfilled a mitzvah, despite the fact that a transgression was committed. This is because the mitzvah is that the child should be circumcised and that result was attained. The mitzvah of yibum is the acquisition of the yevamah through an act of yibum. While one who performs a yibum with a niddah has violated the prohibition of having relations with a niddah, he nevertheless has acquired the yevamah and thus has fulfilled the mitzvah of yibum.
Tosfos asks: Why didn’t the Mishna include a case where the yevamah was a niddah at the time that she fell to yibum? She is forbidden to him at that time and he would be subject to the penalty of kares; wouldn’t there be a prohibition against performing a yibum with her even after she purifies herself?
Tosfos compares this case to the case listed in the Mishna; his wife’s sister. He cannot perform a yibum with her even if his wife would die (one is permitted to marry his wife’s sister after his wife dies). The explanation is: We consider the situation at the time when she falls to yibum; since at that time she was forbidden, she remains prohibited forever. Tosfos’ question is: Why don’t we apply the same logic to a woman who was a niddah at the time that she fell to yibum and just as she is forbidden at that time, she should remain prohibited even after she purifies herself?
Tosfos cites a Gemora in Pesachim (72b) which seems to indicate that a yavam who performs a yibum with a niddah had accomplished a mitzvah, and he has acquired her despite the fact that a transgression was committed.
(Tosfos answers this question by creating a distinction between his wife’s sister, who is only forbidden to him and a niddah, who is forbidden to everyone.)
The Acharonim ask: How can it be that the yavam has fulfilled the mitzvah of yibum by performing a yibum with a niddah; shouldn’t this be considered a mitzvah habaah b’aveirah, a mitzvah that is a result of an aveira that is performed? The positive commandment of yibum cannot override the negative transgression of engaging in relations with a niddah; this transgression is subject to the penalty of kares and cannot be overridden by a positive commandment. There are many Rishonim who maintain that one who performs a mitzvah and simultaneously commits an aveira does not fulfill the mitzvah at all.
Reb Elchonon Wasserman (Kovetz Heoros 11:1) answers: One does not fulfill the mitzvah by a case of mitzvah habaah b’aveirah when the mitzvah is merely an action, such as shaking a lulav or blowing a shofar; he cannot be credited with a mitzvah when that action was an aveira, as well. However, when the mitzvah is more than an action, but rather what the action accomplished, this principle is not applicable. If one circumcises a nine-day old baby on Shabbos, the milah is valid and he has fulfilled a mitzvah, despite the fact that a transgression was committed. This is because the mitzvah is that the child should be circumcised and that result was attained. The mitzvah of yibum is the acquisition of the yevamah through an act of yibum. While one who performs a yibum with a niddah has violated the prohibition of having relations with a niddah, he nevertheless has acquired the yevamah and thus has fulfilled the mitzvah of yibum.
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