מיזקק נמי זקיק:

the husband’s brother also causes a levirate bond with his yevama even if he is a mamzer.

ואחיו הוא לכל דבר: למאי הלכתא ליורשו וליטמא לו

§ The mishna stated that he is his brother in all respects. The Gemara asks: With regard to what halakha was this said? The Gemara answers: To inherit from him, and to become ritually impure for him if he is a priest, as a brother is one of the relations for whom a priest is permitted to become ritually impure by attending his funeral.

פשיטא אחיו הוא סד"א הואיל וכתיב (ויקרא כא, ב) כי אם לשארו הקרוב אליו ואמר מר שארו זו אשתו וכתיב (ויקרא כא, ד) לא יטמא בעל בעמיו להחלו יש בעל שמטמא ויש בעל שאין מטמא הא כיצד מטמא הוא לאשתו כשרה ואין מטמא לאשתו פסולה

The Gemara asks: Isn’t this obvious, as he is his brother? The Gemara answers: It might enter your mind to say that a priest does not become impure for his brother of flawed lineage, since it is written with regard to priests: “None shall defile himself for the dead among his people except for his kin, that is near unto him” (Leviticus 21:1–2), and the Master said “his kin”; this is his wife, and a priest may defile himself for his wife. But it is written: “He shall not defile himself, being a husband among his people, to profane himself” (Leviticus 21:4), implying that he may not defile himself for his wife. This apparent contradiction is resolved as follows: There is a husband who does become impure for his wife, and there is a husband who does not become impure. How so? He becomes impure for his wife if she was fit and was permitted to a priest, but he does not become impure for his wife if she was unfit to marry a priest.

ה"נ מטמא הוא לאח כשר ואין מטמא לאח פסול קמ"ל ואימא ה"נ התם לאפוקי קיימא הכא אחיו הוא:

So too, one might have thought that a priest becomes impure for a brother who is of unflawed lineage and not a mamzer, but he does not become impure for a brother of flawed lineage. Therefore, this comes to teach us that he does become impure for a brother who is a mamzer. The Gemara asks: Say this also, that by the same reasoning by which a priest may not become impure for a wife who was unfit for him, he may not become impure for an unfit brother either. Just as he does not become impure for an unfit wife, he should not become impure for an unfit brother? The Gemara answers: The two cases are different, as there, with regard to the unfit wife, she stands to be dismissed, since according to Torah law unfit women must be divorced. For this reason he does not become impure for her. Here, he is his brother.

חוץ ממי שיש לו אח מן השפחה ומן העובדת כוכבים: מ"ט אמר קרא (שמות כא, ד) האשה וילדיה תהיה לאדוניה:

§ The mishna says: Except for whoever has a brother born from a Canaanite maidservant or from a gentile woman, as in these cases he is not considered a brother. The Gemara asks: What is the reason for this halakha? The Gemara answers: The verse states with regard to a Jewish slave who has children with a Canaanite maidservant: “The wife and her children shall be her master’s” (Exodus 21:4). This means that the children also have the status of Canaanite slaves, which implies that the lineage of a Canaanite maidservant’s child comes from the mother and not from the father.

מי שיש לו בן מ"מ פוטר וכו': מכל מקום לאתויי מאי אמר רב יהודה לאיתויי ממזר מאי טעמא דאמר קרא (דברים כה, ה) ובן אין לו עיין עליו:

The mishna states: With regard to anyone who has a son of any kind, that son exempts his father’s wife from any levirate bond. The Gemara asks: What is added by the phrase: Of any kind? Rav Yehuda said: It adds a child who is a mamzer. What is the reason that a mamzer exempts a woman from any levirate bond? As the verse states with regard to levirate marriage: “And he has no [ain lo] child” (Deuteronomy 25:5). The phrase ain lo can be understood as ayyen alav, meaning investigate him. That is to say, investigate whether he is indeed childless, because any child, even a mamzer, exempts the widow from a levirate bond.

וחייב על מכתו: אמאי קרי כאן (שמות כב, כז) ונשיא בעמך לא תאור בעושה מעשה עמך

It states in the mishna that a child who is a mamzer is liable to receive punishment if he strikes his father or curses him. The Gemara asks: Why should he be liable? Read here the verse: “You shall not curse a ruler of your people” (Exodus 22:27), which is interpreted to imply that the prohibition against cursing applies only to one who acts according to the deeds of your people. However, this father must have engaged in sexual relations with a woman who was forbidden to him in order to have a child who is a mamzer. Therefore, he did not act according to the deeds of the people. In that case, why should this son be liable to receive punishment for hitting him or cursing him?

כדאמר רב פנחס משמיה דרב פפא בעושה תשובה הכא נמי כשעשה תשובה

The Gemara answers: As Rav Pineḥas said in the name of Rav Pappa with regard to a different matter: This is referring to one who performs repentance. Here too, it is referring to one who performed repentance after the mamzer was born. He is thereafter considered to be living in accordance with halakha, and so his mamzer son is liable to receive punishment for cursing him.

תנו רבנן הבא על אחותו והיא בת אשת אביו חייב משום אחותו ומשום בת אשת אביו רבי יוסי בן יהודה אומר אינו חייב אלא משום אחותו בלבד ולא מפני בת אשת אביו

§ The Sages taught: He who unwittingly engages in sexual relations with his sister, who is the daughter of his father’s lawful wife, is liable and must bring two sin-offerings. He must bring one sin-offering because she is his sister and another because she is the daughter of his father’s wife, which is mentioned in a separate verse in the Torah. Rabbi Yosei ben Yehuda says: He is liable only because she is his sister alone, and not because of the prohibition against engaging in sexual relations with the daughter of his father’s wife.

מאי טעמייהו דרבנן אמרי מכדי כתיב (ויקרא יח, ט) ערות אחותך בת אביך או בת אמך ערות בת אשת אביך מולדת אביך אחותך היא למה לי ש"מ לחייבו משום אחותו ומשום בת אשת אביו

The Gemara asks: What is the reason for the opinion of the Rabbis, the first tanna of the baraita? They said: Since it is written in the Torah: “The nakedness of your sister, the daughter of your father, or the daughter of your mother…you shall not uncover” (Leviticus 18:9), then why do I need the verse: “The nakedness of your father’s wife’s daughter, begotten of your father, she is your sister…you shall not uncover” (Leviticus 18:11)? Conclude from this that the additional verse serves to make him liable separately due to the fact that she is his sister and due to the fact that she is his father’s wife’s daughter.

ורבי יוסי בר' יהודה אמר קרא אחותך היא משום אחותו אתה מחייבו ואי אתה מחייבו משום בת אשת אביו

And Rabbi Yosei, son of Rabbi Yehuda, says: The verse states: “She is your sister,” which comes to limit and emphasize that you hold him liable due to the fact that she is his sister but you do not hold him liable a second time due to the fact that she is also his father’s wife’s daughter.

ורבנן האי אחותך היא מאי עבדי ליה מיבעי להו לחייבו על אחותו בת אביו ובת אמו לומר שאין מזהירין מן הדין

The Gemara asks: And what do the Rabbis do with this verse: “She is your sister”? They require it in order to hold him liable to receive punishment for having relations with his sister who is both the daughter of his father and the daughter of his mother, to say that the Sages do not derive an additional Torah prohibition by way of a logical derivation, i.e., an a fortiori inference. In other words, although it would seem that since he is liable to receive punishment for having relations with his father’s daughter and also for his mother’s daughter, this must be all the more true in a case where she is the daughter of both his father and his mother, yet nevertheless, there is no new prohibition here. Therefore, the matter was taught explicitly in the Torah, to teach that the Sages may not derive additional prohibitions by way of a fortiori inferences.

ורבי יוסי בר' יהודה אם כן לכתוב רחמנא אחותך היא למה לי משום אחותך אתה מחייבו ואי אתה מחייבו משום בת אשת אביו

And Rabbi Yosei, son of Rabbi Yehuda, says: If so, that the matter was written for this purpose, then the Merciful One should have written only: Your sister. Why do I need the phrase “she is”? This limits the halakha and teaches that for unintentional intercourse with his sister who is also his father’s wife’s daughter you hold him liable because she is his sister; you do not additionally hold him liable because she is his father’s wife’s daughter.

ורבנן אף על גב דכתיב אחותך איצטריך למכתב היא שלא תאמר בעלמא מזהירין מן הדין

And how do the Rabbis interpret the additional phrase “she is”? Although “your sister” is written, it was necessary to write “she is” so that you not say that generally the Sages may derive additional prohibitions by way of a logical derivation, i.e., an a fortiori inference.

וכ"ת אחותך דכתב רחמנא למה לי מילתא דאתיא בק"ו טרח וכתב לה קרא כתב רחמנא היא

The Gemara explains how this principle is derived from the text. As, if you would say: Why do I need the term “your sister” that the Merciful One writes in the Torah, since it is possible to derive this by a fortiori inference and it therefore appears redundant? The answer is that the Torah wrote this apparently redundant term to teach the principle that with regard to a matter that could be derived by means of an a fortiori inference, the verse unnecessarily wrote it explicitly. The Merciful One writes the term “she is” in order to teach that just as in this case the Sages may not derive additional prohibitions by way of an a fortiori inference, so too, in all other cases they may not derive additional prohibitions that are not stated in the text.

ורבי יוסי בר' יהודה אם כן ליכתביה רחמנא לאחותך היא באידך קרא

And Rabbi Yosei, son of Rabbi Yehuda, says: If this is the reason for the verse, then let the Merciful One write: She is your sister, in the other verse, which states: “The nakedness of your sister, the daughter of your father, or the daughter of your mother,” and not in the verse that speaks of the daughter of one’s father’s wife.

ור' יוסי בר' יהודה האי בת אשת אביך מאי עביד ליה מיבעי ליה מי שיש לו אישות לאביך בה פרט לאחותו משפחה ועובדת כוכבים שאין אישות לאביך בה

The Gemara asks: And what does Rabbi Yosei, son of Rabbi Yehuda, do with this verse, which states: “The daughter of your father’s wife” (Leviticus 18:9)? He requires it to teach that one is liable only for relations with a half-sister whose mother is someone with whom there is no possibility of forging marital ties with your father, even if she was not in actuality his wife. This comes to exclude his sister from a Canaanite maidservant or a gentile woman, with whom your father cannot have marital ties, since betrothal does not take effect with these women.

ואימא פרט לאחותו מאנוסה ההיא לא מצית אמרת מדרבא

The Gemara asks: So say: “Your father’s wife’s daughter” comes to teach specifically the case of a daughter from a woman married to his father, thereby excluding from the definition of: His sister, his sister from a woman who had been raped. Perhaps if she was born of a woman who was not married to his father she would not be considered his sister. The Gemara answers: You cannot say that due to the reason said by Rava.

דרבא רמי כתיב (ויקרא יח, י) ערות בת בנך או בת בתך לא תגלה הא בת בנה דידה ובת בתה דידה שרי וכתיב (ויקרא יח, יז) ערות אשה ובתה לא תגלה את בת בנה ואת בת בתה הא כיצד

As Rava raised a contradiction: In one verse it is written: “The nakedness of your son’s daughter, or of your daughter’s daughter, you shall not uncover” (Leviticus 18:10), which implies, consequently, that her son’s daughter or her daughter’s daughter is permitted. And it is written elsewhere: “You shall not uncover the nakedness of a woman and her daughter; you shall not take her son’s daughter or her daughter’s daughter” (Leviticus 18:17), which states explicitly that her son’s daughter and daughter’s daughter are prohibited. How can these texts be reconciled?

כאן באונסין כאן בנשואין

Here, it is referring to a daughter by a woman whom one raped, in which case he is allowed to marry her daughter provided she is not also his daughter. There, it is referring to a woman whom one married, in which case it is forbidden to marry her daughter under any circumstances. However, even in a case where one’s father raped a woman, one is still permitted to marry the woman’s daughter by another man. If one’s father had a daughter with this woman, one would be prohibited from having a relationship with that daughter, as she is his paternal half-sister.