1) RAV OSHIYA'S EXPLANATION OF REBBI SHIMON'S OPINION
QUESTION: Rav Oshiya states (18b) that Rebbi Shimon permits Yibum in a case of "Nolad v'Achar Kach Yibem": the first brother (Reuven) dies childless, and the second brother (Shimon) has not yet performed Yibum when a third brother (Levi) is born. When Shimon does Yibum with Reuven's wife and then dies childless, Rebbi Shimon permits Levi to do Yibum with Reuven's wife. Rebbi Shimon maintains "Yesh Zikah" -- the bond of Zikah is comparable to the bond of marriage when it bonds the Yevamah to the Yavam. Since the Yevamah was considered "bonded" to Shimon with a marriage-like bond at the time she fell to Yibum, the birth of Levi at that time is not considered a case of "Eshes Achiv she'Lo Hayah b'Olamo."
The Gemara challenges Rav Oshiya's assertion that Rebbi Shimon maintains "Yesh Zikah." The Mishnah later (31b) presents a case of three brothers who were married to three unrelated women. Reuven died, and Shimon did "Ma'amar" with Reuven's wife, and then he died. Rebbi Shimon states that the remaining brother may do Yibum with one widow (either the widow of Reuven or the widow of Shimon) and do Chalitzah with the other. The reason why, according to Rebbi Shimon, the brother may not do Yibum with both of them, or do Yibum with only one of them and exempt the other entirely, is that Rebbi Shimon maintains "Ein Zikah" and he is in doubt whether "Ma'amar" is considered powerful enough to give the Yevamah the status of Shimon's full-fledged wife.
Abaye answers this question by saying that Rebbi Shimon distinguishes between a case of a woman who falls to Yibum to two brothers* and a case of a woman who falls to Yibum to only one brother. There is Zikah when she falls to only one brother, but there cannot be Zikah when she falls to two brothers (just as there cannot be a bond of marriage between one woman and two men).
The Gemara challenges Abaye's distinction between Zikah to two brothers and Zikah to one brother. A Beraisa quotes Rebbi Shimon as saying that if Levi is born before Shimon does Yibum, it is a case of "Eshes Achiv she'Lo Hayah b'Olamo" and Levi is exempt from Yibum and Chalitzah. The Gemara says that according to the simple reading of the Beraisa, the Beraisa refers to a case in which there was only one brother other than Levi, and yet Levi still is considered "Achiv she'Lo Hayah b'Olamo" because there was no Zikah between Shimon and the Yevamah! This proves that even when there is only one brother, Rebbi Shimon maintains "Ein Zikah."
The Gemara answers that the Beraisa actually refers to a case in which there were two other brothers. Hence, Rebbi Shimon says that there is no Zikah, and that is why Levi is considered "Achiv she'Lo Hayah b'Olamo."
Why does the Gemara ask this question from the Beraisa now, in response to Abaye's statement that Rebbi Shimon distinguishes between a case of one surviving brother and two surviving brothers with regard to Zikah? The Gemara should cite this Beraisa at the beginning of the Sugya as a direct challenge to the view of Rav Oshiya! The Beraisa says clearly that Rebbi Shimon agrees -- in a case in which Levi was born before Shimon did Yibum -- that "Ein Zikah" and Levi is prohibited to the Yevamah because of "Eshes Achiv she'Lo Hayah b'Olamo." Before Abaye says that Rebbi Shimon distinguishes between one brother and two brothers, the Gemara should present this Beraisa as an even greater challenge to Rav Oshiya's statement.
ANSWER: The RAMBAN writes that the introductory words which precede the Gemara's question from the Beraisa, "u'Mi Sha'ani Lei l'Rebbi Shimon" -- "Does Rebbi Shimon really distinguish [between one brother and two brothers]?" are not actually the basis for its question from the Beraisa. Although these introductory words imply that the Gemara's question is addressed to Abaye, the Gemara actually intends to ask from the Beraisa directly on Rav Oshiya's explanation of Rebbi Shimon. (The Gemara means to say, "Will differentiating between one and two Yevamim really answer all of the questions on Rav Oshiya?") The question of the Gemara is how can Rav Oshiya say that Rebbi Shimon maintains "Yesh Zikah" when the Beraisa clearly shows that Rebbi Shimon maintains "Ein Zikah."
According to the Ramban's explanation, why does the Gemara not challenge Rav Oshiya's statement from this Beraisa first? After all, this Beraisa is the clearest source that Rebbi Shimon maintains "Ein Zikah." The RASHBA offers several answers:
(a) The reason why this Beraisa is presented as the second question on Rav Oshiya and not the first is that the first question the Gemara asks is from a Mishnah (31b). The general rule is that the Gemara prefers to ask a question from a Mishnah, even when the question is based only on an inference, than to ask a question from a Beraisa, even though the Beraisa is more explicit.
(b) The Rashba suggests further that the first question is stronger because it shows that Rebbi Shimon is in doubt whether or not there is Zikah even after "Ma'amar" is done.
(c) Finally, the Rashba suggests that Rav Yosef -- who asked the first question -- was not aware of the Beraisa. The Gemara (and not Rav Yosef) cites the Beraisa now as an additional question on the opinion of Rav Oshiya.