THE REASON OF R. SHIMON
Question (Mishnah): When he does Yibum, she is like his wife in all respects.
(R. Yosi bar Chanina): This teaches that he can divorce her and remarry her.
Here too, we should say that she still has the status of a Yevamah, and Chalitzah should be required (to divorce her)!
Answer: "u'Lekachah Lo l'Ishah" - once he takes her, she is like his wife in all respects.
Question: It also says "v'Yibmah" (she is still like a Yevamah)! Why do we expound this regarding Eshes Achiv she'Lo Hayah b'Olamo, but not regarding divorce?
Answer: It is reasonable to apply the Heter (to divorce without Chalitzah) to the permitted case (regular Yibum), and the Isur to the forbidden case (Eshes Achiv she'Lo Hayah b'Olamo).
Question: R. Shimon says that since he entered the world when she was permitted, she remains permitted. If: Levi was born after his maternal sister (Dinah) married Shimon's paternal brother (Yosef), and then Yosef died, R. Shimon should permit Yibum, since Levi found her permitted!
Answer: No. The Isur of a maternal sister is there!
Question: Here also, R. Shimon should say that the Isur of Eshes Achiv she'Lo Hayah b'Olamo is there!
Answer: That Isur is sometimes lifted. A maternal sister is never permitted.
WOMEN WHO DO CHALITZAH, AND NOT YIBUM
(Mishnah): The Klal (general rule) is, any Isur of Ervah does neither Yibum nor Chalitzah;
An Isur of Mitzvah or Kedushah does Chalitzah but not Yibum.
Isurim of Mitzvah are Sheniyos (relatives forbidden mid'Rabanan). Isurim of Kedushah are a widow to a Kohen Gadol, a divorcee or a Chalutzah to a regular Kohen, a Mamzeres or Nesinah to a Yisrael, and a Bas Yisrael to a Nasin or Mamzer.
Her sister that is his Yevamah does Chalitzah or Yibum.
(Gemara) Question: What does 'the Klal' come to include?
Version #1 - Answer (Rafram bar Papa): It includes Tzaras Ailonis, like Rav Asi holds.
Version #2: Inference: Regarding Isurim of Ervah the Tzarah is forbidden. In other cases, the Tzarah is permitted.
Question: What does this come to exclude?
Answer (Rafram bar Papa): It includes Tzaras Ailonis, unlike Rav Asi. (end of Version #2)
(Mishnah): Her sister that is his Yevamah...
Question: Whose sister is this?
Suggestion: It is the sister of Isurei Mitzvah.
Rejection: Mid'Oraisa, Isurei Mitzvah fall to Yibum like regular Yevamos, so he may not marry the sister!
Answer: It is the sister of Ervah.
(Mishnah): Isurim of Mitzvah are Sheniyos.
Question: Why are they called Isurei Mitzvah?
Answer (Abaye): It is a Mitzvah to obey the commands of Chachamim.
(Mishnah): Isurim of Kedushah are a widow to a Kohen Gadol...
Question: Why are these called Isurei Kedushah?
Answer: It says about Kohanim "Kedoshim Yihyu (they will observe Isurei Kehunah)".
(Beraisa): R. Yehudah learns oppositely. Isurim of Mitzvah are a widow to a Kohen Gadol...
Question: Why are these called Isurei Mitzvah?
Answer: "Eleh ha'Mitzvos" (refers to all the Mitzvos in Vayikra, including Isurei Kehunah).
Question: Why does R. Yehudah call Sheniyos Isurei Kedushah?
Answer #1 (Abaye): One who fulfills mid'Rabanan Mitzvos is called Kadosh.
Objection (Rava): If one does not keep mid'Rabanan Mitzvos, is he merely not called Kadosh? He is called a Rasha!
Answer #2 (Rava): Mekadesh (sanctify) yourselves through (abstention from) what is permitted to you.
YIBUM OF CHAYVEI LAVIN
(Mishnah): A widow to a Kohen Gadol...
We do not distinguish whether she was widowed from Kidushin or Nisu'in.
Question: We understand this if she was widowed from marriage. A Lav and an Aseh forbid her, and an Aseh (Yibum) does not override them.
However, if she was widowed from Kidushin, only a Lav forbids her. The Aseh should override it (Yibum should be allowed)!
Answer #1 (Rav Gidal): "Yevimto will ascend to the gate" is extra, to teach that there is a Yevamah who does Chalitzah but not Yibum, i.e. Chayavei Lavin.
Question: Perhaps it refers to Chayavei Kerisos!
Answer: "If he does not want" connotes that if he wanted, he could do Yibum. Anyone who could do Yibum (but does not want to) does Chalitzah. Anyone who cannot do Yibum does not do Chalitzah.
Question: If so the same should apply to Chayavei Lavin!
Answer: "Yevimto" includes them among those who do Chalitzah.
Question: (The verse does not specify what to include and exclude.) Why do we include Chayavei Lavin and exclude Chayavei Kerisos, and not vice-versa?
Answer: Kidushin of Chayavei Lavin takes effect, so it is more reasonable to include them than Chayavei Kerisos.
Question (Rava - Beraisa): If one did Yibum or Chalitzah with Isurei Mitzvah or Isurei Kedushah, the Tzarah is exempted.
If mid'Oraisa Chayavei Lavin do Chalitzah but not Yibum, why does Yibum exempt the Tzarah?
Answer (Rava): The Beraisa taught separate laws together;
Yibum of Isurei Mitzvah, or Chalitzah of Isurei Kedushah, exempts the Tzarah.
Question (Rava - Beraisa): If Reuven is a Petzu'a Daka, Krus Shofchah or Seris Adam (castrated, and hence forbidden to marry), or if he is too old (to have children), he does Chalitzah or Yibum (Rashi - Yibum is Asur, but if done it acquires; Hagahos ha'Gra - he does Chalitzah but not Yibum);
If his widow fell to Yibum and his brother Shimon gave a Ma'amar or Get or did Chalitzah, it takes effect. If he did Yibum, he acquires her;
If Shimon died and Reuven gave a Ma'amar or Get or did Chalitzah, it takes effect. If he did Yibum, he acquires her, but they may not remain married - "Lo Yavo Fetzu'a Daka ... (bi'Khal Hash-m)".
If mid'Oraisa, Chayavei Lavin do Chalitzah but not Yibum, why does he acquire her through Yibum?
Answer #2 (to Question (c) - Rava): Also a widow from Kidushin is forbidden to a Kohen Gadol by an Aseh (as well as a Lav), "Kedoshim Yihyu."
Question: How can we answer for a Mamzeres or Nesinah?
Answer: There is an Aseh (even for Yisraelim) "V'Hiskadashtem".
Objection: If so, every Lav in the Torah is also an Aseh!
Answer #3 (Rava): Chachamim decreed that a widow from Kidushin may not do Yibum, lest a widow from Nisu'in do Yibum.
Question: How can we answer for Mamzeres or Nesinah?
Answer: Chachamim forbade them to do Yibum, lest one marry them Stam (without the Mitzvah of Yibum).
Objection: If so, we should forbid a paternal brother to do Yibum, lest a maternal brother (marry Eshes Achiv, and) think that he does Yibum!
Answer: People will not confuse these. They know that Yibum depends on inheritance.
Objection: We should forbid a woman without children to do Yibum, lest a woman with children do Yibum!
Answer: People will not confuse these. They know that Yibum depends on (not having) children.
Objection: We should forbid Eshes Achiv she'Hayah b'Olamo to do Yibum, lest Eshes Achiv she'Lo Hayah b'Olamo do Yibum!
Answer: People will not confuse these. They know that Yibum depends on dwelling together in the world.
Objection: No woman should do Yibum, lest an Ailonis do Yibum!
Answer: An Ailonis is uncommon, so we do not make such a decree.
Objection: Mamzeres and Nesinah are also uncommon (yet we decreed)!
Answer #4 (Rava): We forbid Yibum lest he have a second Bi'ah with her (which is forbidden).
Support (Beraisa): If they did Yibum he acquires her with the first Bi'ah. A second Bi'ah is forbidden.
Retraction (and Answer #5 - Rava): My answer is wrong. Reish Lakish taught that an Aseh overrides a Lav only if one cannot fulfill both;
Here, both can be fulfilled through Chalitzah!
Question (Beraisa): If they did Yibum, he acquires her.
Answer #5 is refuted.