Gemara

(Mishnah) Question: What is the case of Eshes Achiv she'Lo Hayah b'Olamo (the widow of a brother who died before the Yavam was born)?

Answer: Reuven and Shimon are brothers. After Reuven died, Levi (another brother) was born. Then, Shimon did Yibum with Chanah (Reuven's widow), and Shimon died;

The first wife is exempt because her husband was not alive in the life of the Yavam; the second wife (of Shimon, Leah,) is exempt because she is Chanah's Tzarah.

If Shimon merely gave a Ma'amar to Chanah, Leah does Chalitzah but not Yibum.

(Gemara) Question: Where does the Torah discuss Eshes Achiv she'Lo Hayah b'Olamo?

Answer (Rav Yehudah): "Ki Yeshvu Achim Yachdav" - they dwelled together in the world;

18a Question (Rav Huna bar Chiya - Mishnah): If he gave her a Ma'amar and died, the second widow does Chalitzah but not Yibum.

Inference: Had he not given a Ma'amar, the second widow could do even Yibum.

If Yesh Zikah, she should be like the Tzarah of Eshes Achiv she'Lo Hayah b'Olamo (which is forbidden)!

Answer (Rabah): No, even if no Ma'amar was given, she may not do Yibum. The Mishnah discusses a Ma'amar to oppose Beis Shamai, who say that a Ma'amar fully acquires a Yevamah.

18b (Mishnah): Reuven died, and Shimon did Yibum with Chanah. Levi (another brother) was born, and Shimon died. Chanah is exempt, for she is Eshes Achiv she'Lo Hayah b'Olamo. Leah (Shimon's other wife) is exempt, because she is Chanah's Tzarah.

If Shimon gave a Ma'amar and died, Leah does Chalitzah but not Yibum;

R. Shimon says, Levi may do Yibum or Chalitzah with either woman.

(Gemara - Rav Oshaya): R. Shimon argues even in the previous Mishnah (when Levi was born before Yibum was done).

The Reisha is not needed to teach about Chachamim. They forbid even when Levi was born after Yibum and entered the world when Chanah was permitted, all the more so if he was born before Yibum! Rather, it teaches that R. Shimon argues even in such a case.

Our Mishnah teaches the extremity of Chachamim (they forbid even when Levi was born after Yibum).

Question: According to R. Shimon, when is Eshes Achiv she'Lo Hayah b'Olamo forbidden?

Answer #1: This is when a man dies without brothers, and then a brother is born.

Answer #2: This is when there were two brothers, one died, a third brother was born and the surviving brother did not do Yibum or die.

Question: We understand why R. Shimon permits when Levi was born after Yibum. When he entered the world, Chanah was permitted. But when he was born before Yibum, why may he do Yibum?

(Rav Papa): R. Shimon argues only when the brother is born after Yibum. R. Shimon agrees that if he was born before, she is exempt. Both Mishnayos are needed to teach the opinion of Chachamim. The second Mishnah teaches more than the first.

Support (for Rav Papa and refutation of Rav Oshayah - Beraisa): Reuven died, leaving a brother Shimon. Levi (another brother) was born before Shimon had a chance to give a Ma'amar, then Shimon died. The first widow (Reuven's) is exempt due to Eshes Achiv she'Lo Hayah b'Olamo. The second (Shimon's other wife) does Chalitzah or Yibum;

If Shimon gave a Ma'amar before or after Levi was born and then Shimon died, the first widow is exempt due to Eshes Achiv she'Lo Hayah b'Olamo. The second does Chalitzah, not Yibum;

R. Shimon says, Yibum or Chalitzah with one woman (i.e. the one who did not receive the Ma'amar) exempts her Tzarah. If Ba'alas ha'Ma'amar (the one who received the Ma'amar) does Chalitzah, her Tzarah is not exempted.

R. Meir says, if Shimon did Yibum and died, and then Levi was born, both women are exempt from Chalitzah and Yibum. The same applies if Levi was born and then Shimon did Yibum and died;

R. Shimon says, since he (Levi) entered the world when she (the first widow) was permitted, and she was never forbidden to him, he may do Yibum or Chalitzah with either one.

The last clause does not teach about R. Meir's opinion. He does not distinguish whether Levi was born before or after Yibum. R. Meir could teach both cases together!

Rather, it teaches about R. Shimon, that he argues only when the Yibum preceded the birth. He admits when the birth preceded the Yibum.

Rishonim

The Rif and Rosh (2:1): bring the Mishnayos and Rav Yehudah's source for Eshes Achiv she'Lo Hayah b'Olamo.

Nimukei Yosef (DH Keitzad): We hold like our Stam Mishnah, for it is the majority opinion. Eshes Achiv she'Lo Hayah b'Olamo applies whether the last brother was born before or after Yibum.

Rosh (ibid.): We hold that Yesh Zikah, even without a Ma'amar. Therefore, the second widow may not do Yibum, for she is like Tzaras Eshes Achiv she'Lo Hayah b'Olamo.

Rambam (Hilchos Yibum 6:16): "Ki Yeshvu Achim Yachdav" teaches that there is no Zikah on Eshes Achiv she'Lo Hayah b'Olamo, therefore she (is exempt and) exempts her Tzarah.

Rambam (17): The case is when Reuven died, and his wife Leah fell to Shimon. Levi was born after Reuven died, before or after Shimon did Yibum. Leah is permanently Ervah to Levi. Therefore, if Shimon died (after doing Yibum), Le'ah and Shimon's other wife are exempt from Chalitzah and Yibum.

Rambam (18): If Shimon gave a Ma'amar to Leah and died before doing Yibum, Levi does Chalitzah to Levi's Tzarah. He may not do Yibum, because Ma'amar does not acquire totally (therefore, Chalitzah is required).

Question (Magid Mishneh): The Gemara (18a) says that if Yesh Zikah, even without a Ma'amar, the second widow may not do Yibum, for she is like Tzaras Eshes Achiv she'Lo Hayah b'Olamo! The Mishnah discusses a Ma'amar to oppose Beis Shamai, who say that it fully acquires. Since we hold that Yesh Zikah, why did the Rif and Rambam omit this? The Rashba brings this Halachah.

Answer (Kesef Mishneh 10): Elsewhere, the Rif (8a) and Rambam (Halachah 25) say (regarding different Mishnayos) that Yesh Zikah even without a Ma'amar, and this forbids Yibum, for she is like Tzaras Ervah. The Mishnah discusses a Ma'amar to oppose Beis Shamai. This applies to all similar cases, including Eshes Achiv she'Lo Hayah b'Olamo.

Poskim

Shulchan Aruch (EH 173:12): Eshes Achiv she'Lo Hayah b'Olamo is Eshes Ach without the Mitzvah of Yibum, so she exempts her Tzarah. I.e. Reuven died, and his wife Leah fell to Shimon. Levi was born after Reuven died, before or after Shimon did Yibum. Leah is permanently forbidden to Levi. Therefore, if Shimon died (after doing Yibum) leaving another wife, also she is exempt from Chalitzah and Yibum. She exempts her Tzarah and Tzaras Tzarah ad infinitum, like Tzaras Ervah.

Beis Yosef (DH u'Mah she'Chosav Bein): The Rif (Shabbos 48a) and Tosfos (Yevamos 42b) say that the Halachah follows Chachamim.

Note: They do not discuss our Mishnah in particular. They merely say that the Halachah follows the Stam opinion. We do not call our Mishnah a Stam followed by a Machlokes, because also in the Seifa the opinion of Chachamim is anonymous.

Shulchan Aruch (ibid.): If Shimon died before even giving a Ma'amar, his wife does Chalitzah, not Yibum. Since we hold that Yesh Zikah even without a Ma'amar, she is like Tzaras Eshes Achiv she'Lo Hayah b'Olamo.

Beis Shmu'el (15): Shiltei ha'Giborim (3b:3) says that we hold that mid'Rabanan, Yesh Zikah. R. Chananel (brought in Tosfos 27b DH Aval) holds that according to the opinion that Yesh Zikah, it is mid'Oraisa.