In general, it is forbidden for a man to marry a woman who was married to his brother. The exception is in the context of yibum; where the brother passed away without having any children. He must either perform yibum thereby marrying her -- the yavama -- or chalitzah is performed through which she is permitted to marry someone else. The masechet begins by teaching that this is true unless the yavama is already forbidden to him due to another familial relationship. One example is if the widow is his daughter. The Mishnah explains that even if the deceased man had two wives and only one of them was a forbidden relationship, the other wife, the tzara, would also not require yibum or chalitza.
One of the forbidden relationships listed is an eshet achiv she'lo hayah be'olamo. The second perek opens by explaining the case. One way is if there were a number brothers, but one was born after the husband died and before was yibum performed. The brother born after the death is defined as eshet achiv she'lo hayah be'olamo and is forbidden to perform yibum. The Bartenura (1:1) explains that the Torah excluded this case from the mitzvah of yibum. When the Torah teaches the mitzvah, it begins, "when two brothers 'dwell' together..." (Devarim 25:5). The Gemara (17b) explains that they must be dwelling in the world, i.e. alive at the same time, for the mitzvah to apply. Consequently, once the young brother is born, for him, the yavama is if considered as if they had children. In other words, the permit of yibum does not apply and the regular prohibition of marrying one's brother's wife is in place.
The second case, which is the subject of debate, is where the young brother was born after the older brother performed yibum. After the young brother was born, the older brother then died. The Chachamim understand that this is also defined as eshet achiv she'lo hayah be'olamo where as R' Shimon disagrees. How do we understand this debate?
According to the Chachamim this case is no different. The widow is the first brother's wife that we never permitted to him through yibum, because he was born only after the first brother died. According to R' Shimon however, the fact that yibum was performed before the young brother was born makes a difference. Why?
The Bartenura explains that since yibum was already perform, it means the zika from the first brother, the connection that obligates yibum, is no longer present. The expression the Gemara (20a) uses is that the when he was born, he found that yavama in a state of heiter and not is a state of issur.
Rashi (20a) explains that the second brother was permitted to the yavama due to the mitzvah of yibum. For him, the prohibition of eshet achiv did not apply. When the second brother dies, the younger brother, is able to perform yibum by way of the heter that the second brother employed. The Kovet Ha'arot (4:14) explains that yibum is different to marriage. In yibum the ishut (marriage) is transferred from the late husband to the brother, including the issurim and heteirim associated with that kinyan. Consequently, in this case, the heter would be transferred to the third brother also.
The Tosfot (20a, s.v hai) however explain that when the second brother performed yibum, the prohibition of eshet ach, was completed removed. The proof is that if the second brother divorced her, he would be able to remarry here. That begin the case, the prohibition is removed from everyone, and the third brother would be able to perform yibum after the second brother dies.
Considering both Rashi and Tosfot, why does R' Shimon agree that if the third brother was born prior to the second brother performing yibum, if the second brother dies, he is not be able to perform yibum? The logic, that the third brother can use the heter of his brother should apply?
Rashi (18b, s.v. beheteira) explains that when the brother is born after yibum, yibum is permitted since "there was not a moment when she was assur". This implies that if he was born prior to yibum, since at the time of the zika she was assur, that issur remains, even if the second brother performs yibum. In other words, we are not just dealing with the prohibition of marrying one's brother's wife and whether in the context of yibum it is alleviated. If the brother is born, while she is a shomeret yabam since in this instance (nefillah) she is assur to him, an additional issur now exists directly between them, that cannot be circumvented by trying to use the heter of the second brother (see Rashi, 9b).
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