Gemara

(Reish Lakish): If one was Mekadesh his Yevamah after Chalitzah, his Isur to her has no (punishment of) Kares, but the brothers' Isur has Kares. There is Kares for the Isur of him and his brothers to a Tzarah;

(R. Yochanan): There is no Kares for him or the brothers, not for her or for the Tzaros.

Reish Lakish learns from "That will not build the house of his brother" - once he decided not to build (and did Chalitzah), he may not build.

This applies (only) to the Choletz (the Yavam who did Chalitzah). His brothers are forbidden like they used to be (there is Kares without a Mitzvah of Yibum to permit it). This applies only to the Choletzes (who did Chalitzah). Her Tzaros are forbidden like they used to be.

R. Yochanan disagrees, for we never find that initially any brother may do Chalitzah to any widow, and now (that he did not do Chalitzah to her), he is forbidden to her with Kares. Rather, the Choletz is a Shali'ach on behalf of all the brothers, and the Choletzes is a Shali'ach on behalf of her Tzaros.

Question (R. Yochanan - Beraisa): If one did Chalitzah to his Yevamah, then was Mekadesh her and he died, she must do Chalitzah with the brothers.

This is like me. Since she is only Chayavei Lavin to the brothers, Chalitzah is needed. You say that she is Chayavei Kerisos to the brothers. If so, why is Chalitzah needed?

Counter-question (Seifa): If one of the brothers was Mekadesh her, she has no claim against him (i.e. the Kidushin did not take effect).

If it is only Chayavei Lavin, Kidushin should take effect!

Answer #1 (Rav Sheshes): The Seifa is like R. Akiva, who says that Ein Kidushin Tofsin (Kidushin does not take effect) with Chayavei Lavin.

Objection: If so, it should say that according to R. Akiva, she has no claim against him! (The Reisha cannot be R. Akiva, for if so Ein Kidushin Tofsin even if the Choletz was Mekadesh her.)

This is left difficult.

Rav Ashi holds like Reish Lakish, and answered the question according to R. Shimon. Ravina holds like R. Yochanan, and answered the question like Chachamim;

Answer #2 (Rav Ashi): The Reisha said that if the Choletz was Mekadesh the Choletzes (and he died), she needs Chalitzah from the brothers. This refers to brothers who were born later. It is like R. Shimon (who permits Eshes Achiv she'Lo Hayah b'Olamo. She may not do Yibum, for she was forbidden to her husband, all the more so to her Yavam.)

The Seifa says that if a brother was Mekadesh her, she has no claim against him. This refers to a brother who was already born, like Reish Lakish taught.

Answer #3 (Ravina): The Reisha says that if that if the Choletz was Mekadesh the Choletzes, she needs Chalitzah from the brothers. This refers to brothers that were already born, like R. Yochanan taught.

The Seifa says that if a brother was Mekadesh her, she has no claim against him. This refers to brothers who were born later. It is like Chachamim (who argue with R. Shimon).

(Rav Acha or Ravina): If a man did Yibum, then a brother did Yibum with a Tzarah, the second marriage is forbidden with Kares;

(The other of Rav Acha and Ravina): It is only Chayavei Aseh.

The opinion that is Mechayev Kares holds like Reish Lakish. The opinion that it is only an Aseh holds like R. Yochanan.

40a (Mishnah): If a Yavam did Chalitzah, he is forbidden to her relatives, and she is forbidden to his.

Question: Did Chachamim decree Sheniyos regarding a Chalutzah?

Did they forbid Shniyos only of Ervah mid'Oraisa, but not regarding a Chalutzah?

Or, did they not distinguish (and decreed even regarding a Chalutzah)?

(Shmuel): If one had Bi'ah with the Tzarah of his Chalutzah, the child is a Mamzer.

This is because she retains the Isur of Eshes Ach.

Support (and question against R. Yochanan - Rav Yosef - Mishnah): A man is permitted to the relative of Tzaras Chalutzaso.

If the Tzarah is like the Chalutzah, he should be forbidden to her sister (just like he is forbidden to Achos Chalutzaso)!

Rejection: Achos Chalutzaso is not forbidden mid'Oraisa. (Even if the Tzarah is like the Chalutzah, we can permit the Choletz to her sister.)

Rishonim

Rif and Rosh (13b and 4:21): The Halachah follows R. Yochanan against Reish Lakish, except for three places (this is not among them). When Rav Acha and Ravina argue, the Halachah follows the lenient opinion (it is always Ravina), and all the more so here that the lenient opinion is like R. Yochanan. The Halachah does not follow Shmuel.

Rebuttal (Ra'avad, cited in Sefer ha'Zechus): The Kelalim that the Halachah follows R. Yochanan against Reish Lakish, and Ravina against Rav Acha, do not apply when other Amora'im also argue. Shmuel holds like Reish Lakish, so the Halachah follows them (the Rabim) against R. Yochanan. Rav Ashi holds like Reish Lakish, and the Halachah follows Rav Ashi against Ravina.

Defense (Sefer ha'Zechus): Reish Lakish is supported by Beraisos in the three places where the Halachah follows him against R. Yochanan. This implies that there are only three even counting when others support him. Likewise, (except for three Halachos listed on Pesachim 74b) we follow Ravina against Rav Acha even when others hold like Rav Acha. Likewise, the Kelal that the Halachah follows Rava against Abaye (except for YALKGM) applies even when others argue, and causes us to rule (atypically) like R. Yochanan against R. Yehoshua ben Levi and like Levi against Rav. BaHaG says that even though we follow Rav Ashi (who holds like Reish Lakish) against Rava (who said that there are only three such places), here Ravina holds like R. Yochanan, so we follow him.

Rambam (Hilchos Yibum 1:12): If Reuven did Yibum, the Tzaros become forbidden to him and his brothers. If one of them has Bi'ah with a Tzarah he transgresses an Aseh "Yevamah Yavo Aleha" - not her and a Tzarah. A Lav derived from an Aseh is like an Aseh. Similarly, if Reuven did Chalitzah with Leah, she and the Tzaros become forbidden to him and his brothers. They are forbidden mid'Rabanan, like Shniyos. Once his brother died without children, the Isur Ervah of all his wives vanished. Therefore, Kidushin takes effect on them like Shniyos.

Magid Mishneh: The Rambam holds that according to R. Yochanan, "Lo Yivneh" is a mere Asmachta. The verse does not address this (Yibum after Chalitzah); the Isur is only mid'Rabanan. The Rambam learns from 40b. The Gemara asked whether Chachamim decreed to forbid Shniyos of a Chalutzah, or only of Ervah mid'Oraisa. This implies that a Chalutzah is only mid'Rabanan. However, this is difficult. Even according to R. Yochanan there is a Lav (10b), and according to R. Akiva Ein Kidushin Tofsin (53a). This shows that it is a real Lav! Rather, we ask (40b) whether we forbid Shniyos only of Arayos whose relatives are forbidden mid'Oraisa, or even of a Chalutzah.

Aruch ha'Shulchan (EH 162:4): The Rambam was bothered by Tosfos' question, that the verse is needed to forbid more than one Yibum, so it cannot be the source of an Isur Aseh or Lav. When R. Yochanan states his opinion, he says 'there is no Chiyuv Kares' (Yerushalmi - there is no Chiyuv). He does not say that there is a Lav.

Note: R. Yochanan does say 'I say that it is Chayavei Lavin...' - perhaps he means that it is no more severe than Chayavei Lavin. Chelkas Yakov (YD 217:6 DH v'Hinei) derives from the Shach (YD 145:1) that the Yerushalmi is more authoritative about R. Yochanan's opinion than the Bavli. However, elsewhere the Shach himself (CM 22:6) cites the Lechem Mishneh (Sanhedrin 7:2) who says that we rely on the Bavli's version of R. Yochanan.

Aruch ha'Shulchan (4,5,6): If R. Yochanan held that there was an Isur, the Gemara would have no basis to say that Rav Acha and Ravina argue like Reish Lakish and R. Yochanan. Perhaps the Rambam agrees that there is a Lav, but not 'Derech Binyan' (like Yibum). Through Kidushin it is forbidden only mid'Rabanan. Even the Tur (who says that an Aseh forbids her) could agree with this.

Poskim

Shulchan Aruch (EH 162:1): If Reuven did Yibum, the Tzaros become forbidden to him and his brothers by an Aseh.

Gra (1): Rashi says that the Aseh is "Asher Lo Yivneh Es Beis Achiv" - he may build one house, not two (44a). Tosfos challenges this, because the verse discusses not building. (Indeed, we learn that one may not do Yibum with two women, but we cannot derive an Isur Aseh!) The Rambam learns from "Aleha" - not her and a Tzarah. This is difficult, for the Gemara forbids Yibum with two women from 'Bayis Echad'. Tosfos' text says that they are forbidden by a Lav "Lo Yivneh".

Shulchan Aruch (2): If Reuven did Chalitzah with Leah, she and the Tzaros become forbidden to him and his brothers by an Aseh.

Question (Beis Shmu'el 2): I do not know what Aseh this is. Tosfos and the Ramban say that there is a Lav, and the Rambam says that they are forbidden mid'Rabanan!

Answer #1 (Taz 2): Here also they are forbidden by "Lo Yivneh", a Lav derived from an Aseh.

Answer #2 (Aruch ha'Shulchan 2): Perhaps we learn from "Beis Chalutz ha'Na'al" that he does Chalitzah to only one house. Since there is no Isur in an extra Chalitzah, this must refer to another act (i.e. Yibum) after Chalitzah.

Lechem Mishneh (Hilchos Yibum 1:12) and Gra (2): The correct text of the Tur should say 'a Lav'.