The mishnah on Daf32A says that if 2 bothers married 2 sisters, and then the 1 brother dies (obviously, his widow is free to the world), and then later on her sister dies, that the widow remains Assur to her Yavam, even though the Ervah is no longer alive.
My problem is..
How can we possibly have thought to bring her back. As soon as her husband died, and his brother is married to her sister, she is free to do as she pleases. For all we know, she went off and married another man. How can the Mishnah even have the Hava Aminah that she would have to come back to her Yavam if her sister dies.
(I understand the connection to the earlier Mishand, where there was a third brother, and why there might be a Hava Aminah, that she could be Permitted to her (dead) sister's husband. My point is on this second Mishnah alone)
I do not understand why we even need the concept of "since she was Assur at one time, then she is Assur forever"
Yoni Sudwerts, Yerushalayim
Below is what we wrote on this issue in our Insights to the Daf.
INSIGHTS TO THE DAF
1) PERFORMING YIBUM AFTER AN "ISUR ERVAH" IS REMOVED
OPINIONS: The Mishnah describes a case in which a Yevamah was forbidden to one of the brothers of her husband, and then the prohibition was removed. The Mishnah states that since she was forbidden to that brother for one moment, she remains forbidden to him forever.
The Mishnah refers to a case in which two brothers (Reuven and Shimon) are married to two sisters (Leah and Rachel), and a third brother (Levi) is married to an unrelated woman. When Reuven dies childless, his wife, Leah, falls to Levi for Yibum. She may not do Yibum with Shimon because she is an Ervah to him ("Achos Ishto," the sister of his wife, Rachel). After Levi does Yibum with Leah, the wife of Shimon dies. Then, Levi dies childless. Although Shimon's wife died and thus Leah is no longer an Ervah of "Achos Ishto" to Shimon, Leah nevertheless remains forbidden to him because for one moment -- at the time she first fell to Yibum -- she was an Ervah to him, and thus she remains an "Eshes Ach she'Lo b'Makom Mitzvah."
Rav Yehudah in the name of Rav says that the same Halachah applies during the first Nefilah to Yibum. If, at the time the woman fell to Yibum for the first time, the Yavam was married to her sister, then even if the sister dies afterwards the woman remains exempt from Yibum. The Gemara says that one might have thought that there is a difference between when there are other brothers who can do Yibum with the Yevamah and when there are no other brothers except the one to whom the Yevamah is an Ervah. When there are other brothers, one might have thought that she may do Yibum with Shimon if her sister, Shimon's wife, dies.
Why does the presence of other brothers give more reason to assume that the Yevamah may do Yibum with her sister's husband after her sister dies?
(a) RASHI implies that the logic for the difference is that since there is an existing Zikah (due to the presence of the other brother to whom the woman is Zekukah), when the brother to whom she was forbidden until now becomes eligible for Yibum with her, the Zikah extends to that brother as well. In contrast, when -- at the time of the death of her husband -- there was no other brother to whom Zikah applied, Zikah cannot be created from nothing when her prohibition to the surviving brother is removed.
(b) Another explanation may be suggested based on the words of TOSFOS earlier (2a, DH v'Achos Ishto). According to Tosfos, Zikah may take effect either at the moment the husband dies or at any time after he dies. The Mishnah here asserts that his wife remains forbidden to the brothers forever when she was forbidden at the moment she fell to Yibum not because Zikah cannot take effect after the moment of her husband's death. Rather, the reason is that "Deracheha Darchei No'am" -- the ways of the Torah are ways of pleasantness, and it would not be pleasant for a woman who was exempt from Yibum (and was permitted to remarry) to become obligated to do Yibum (and need to separate from her husband if she already remarried) at a later time. For this reason, if a Yevamah was forbidden to the Yavam at the time of her husband's death, she remains forbidden to him regardless of any eventuality, and she is permitted to marry others. (The Gemara mentions the logic of "Darchei No'am" later (87b) with regard to a case in which a man died and left behind a child, but then the child died. Since, at the time of the man's death, there was no obligation for his widow to do Yibum (because he left behind living offspring), his wife cannot become obligated to do Yibum when the child dies. Once she was already permitted to marry someone else, she remains permitted.)
According to this approach, the Gemara here is clear. If "Deracheha Darchei No'am" is the reason why she does not fall to Yibum when the Isur Ervah to Shimon is removed, then one might have thought that such logic applies only when she was exempt from Yibum entirely (such as when there was only one surviving brother to whom she was forbidden with an Isur Ervah) and she was permitted to marry others. However, in a case in which she was not exempt from Yibum because there was another brother to whom she was not an Ervah, there is no unpleasantness involved if a second brother is required to perform Yibum, even if he was forbidden to her when she first fell to Yibum. This does not involve a lack of "Darchei No'am" because, anyway, she was not permitted to remarry until she did Yibum or Chalitzah with the other brother.
Rav teaches that even in such a case, requiring her to do Yibum with the brother to whom she was originally forbidden is considered a lack of "Darchei No'am." It indeed is unpleasant for a woman to be required to do Yibum with a brother with whom she thought all ties were severed. (The prohibition in such a case because of "Darchei No'am" does not mean that she is not prohibited with an Isur of "Eshes Ach" to that brother. Rather, it means that it is logical that the Torah would not require her to fall to Yibum in such a case, and thus the Isur of "Eshes Ach" remains and the Zikah does not take effect.)
(According to Tosfos' explanation, there are two different reasons for why a Yevamah remains forbidden forever after the first Nefilah and why she remains forbidden forever after the second Nefilah. Her prohibition during the first Nefilah is due to the logic of "Darchei No'am" -- she cannot be required to do Yibum with a man with whom she had no obligation of Yibum. This logic, however, does not prohibit her to that Yavam when she falls to him from a second Nefilah, since there is an entirely new cause for her to fall to him. Rather, during the second Nefilah she is forbidden to him because she is his "Eshes Ach she'Lo b'Makom Mitzvah" from the first Nefilah, and thus she can never become permitted to the Yavam. This approach is problematic, however, because the Gemara later (32a) seems to associate the two Isurim. The Gemara there says that once the Mishnah teaches that a Yevamah who was once forbidden remains forbidden after the second Nefilah, it is obvious that if she was once forbidden she should remain forbidden during the first Nefilah.) (M. Kornfeld)<<