How are we to understand the היתר and מצוה of אשת אח to perform Yibum [normally one is strictly forbidden from marrying one's sister in law - אשת אח. Only if his brother died childless is this union not only permitted but a mitzva]?
A] It is אסור but the Torah permitted it במקום מצוה.
B] If the brother had no children there is no איסור anymore. So it is not that the mitzva permits the איסור but that there is no סיבת האיסור to begin with. Until now the קורבה [relation] created an איסור but now the death of the brother removed any such איסור [for other עריות death doesn't do that so the איסור remains in place either because she is still connected to her husband after death or because their connection during his lifetime created an איסור that doesn't go away]. The death of the brother is a סיבה to connect him to his אשת אח.
From the Gemara at the beginning of Yevamos it is implicit like A] because we try to derive a היתר for all other עריות for the sake of Yibum from the היתר of אשת אח. Now סברא B] doesn't apply to other עריות [if she is one's daughter, for example, how can the death of the brother be a סיבה that she is no longer his daughter??] so it must be that the Gemara is understanding like A]. It is אסור to be with one's sister in law but במקום מצוה it is permitted. And the Gemara tried to learn that just like the איסור of אשת אח is permitted for Yibum, let's permit all איסור עריות.
However, from the Yerushalmi [Yevamos 1-1] it would appear that there is a machlokes about this very point.
Says the Yerushalmi:
Instead of learning all the עריות from אחות אשתו to forbid Yibum, learn from אשת אח to permit Yibum. Rav Mana says that we learn two איסורים from two איסורים [all עריות eligible for Yibum are אסור because of the specific ערוה and also אשת אח - so they should be derived from אחות אשתו who is also אשת אח - two איסורים].
R' Lezer says that any איסור that comes due to a cause, if the cause is cancelled, there is no איסור. If the איסור doesn't come because of a specific cause, if the cause is cancelled, the איסור is not.
Cryptic.
Some Meforshim say that after בטל הגורם, the cause [of איסור] is removed, means that the brother died. This is difficult. אשת אח is אסור even after the brothers death!
Explains Rav Avraham Yitzchak Bloch ztz"l H"yd [his lips are moving in the grave...]: Rav Mana holds like A], so there is a מציאות וסיבת איסור. She is his brother's wife and thus אסורה. Just that the mitzva of Yibum permits this union [the "state of the union" address would then be a loooong pilpul in Yevamos]. Mazel Tov. So other עריות where there is a סיבת איסור can be compared to אשת אח. But למעשה we can't derive other עריות which involve two איסורים to אשת אח which is just one.
Rav Lezer says NO! When the brother dies childless and the mitzva of Yibum applies there is no longer a סיבת האיסור of אשת אח. Thus, we can't compare that case to other עריות where there is still remains a סיבת איסור [the איסור עריות] even after the brother dies. So the "גורם" of איסור אשת אח is no longer extant, but the איסור still applies because of the other עריות problems.
A proof for Rav Mana: Leah is married to Reuvey and he dies. She married his brother Shimmy and he also goes. She then can marry Levi. If we say like side A] then we understand. The mitzva of Yibum permits the איסור of אשת אח [and שם איסור חד הוא as we saw in a previous post]. But if we say like B] then I get how she is no longer forbidden because she was married to Shimmy. He was the סיבת איסור of her marrying Levi and now the סיבה disappeared. But there is still the סיבת האיסור of her previous union with Reuvy and that doesn't become permitted when Shimmy dies...
[עפ"י דברי הגאון מהרא"י בלוך זצ"ל הי"ד עפ"י הבנתי הדלה וכל טעות צריכה להיתלות בי ובי בלבד ]
