Makkot 27
כדאמרינן מעיקרא שאם עשו תשובה ב"ד של מעלה מוחלין להן מאי אמרת הא לא עבוד תשובה לא פסיקא מילתא לכרת
[the dispute] is as we said previously, that [Rebbi Akiva's reason for giving malkus by liabilities of kares is that] if he repents the heavenly court will forgive him. What did you ask? That he hasn't yet repented? Kares is not definite. [He can still repent.]
רבי יצחק אומר חייבי כריתות בכלל היו ולמה יצאת כרת באחותו לדונו בכרת ולא במלקות
Rebbi Yitzhak says, Liabilities of kares were included [in the Torah's prohibition of malkus], only why was kares repeated by [the prohibition of cohabiting with] his sister [kares b'achoso]? To punish him with kares and not malkus.
ורבנן כרת באחותו למה לי לחלק וכדרבי יוחנן דאמר רבי יוחנן שאם עשאן כולם בהעלם אחד חייב על כל אחת ואחת
And why was kares repeated by [the prohibition of cohabiting with] his sister according to the Rabbis [Rebbi Yishmael and Rebbi Akiva]? To [teach us that sacrifices are] separated [with regards to bringing a sacrifice for an inadvertent offence], as Rebbi Yochanon said that if a person did all [of these kares prohibitions] in one lapse of unawareness he is liable [to bring a chatos sacrfifice] for each and every one of them.
ורבי יצחק לחלק מנא ליה נפקא ליה (ויקרא יח, יט) מואל אשה בנדת טומאתה לחייב על כל אשה ואשה
And from where does Rebbi Yitzchak [who uses kares b'achoso to teach us that kares is not subject with malkus] derive the principle of separating? He derives it from (Leviticus 18,19) "And to a woman (אשה) impure by her uncleanliness", to make one liable for each and every woman.
ורבנן נמי תיפוק ליה מהא אין הכי נמי ואלא כרת דאחותו למה לי לחייבו על אחותו ועל אחות אביו ועל אחות אמו
And let the Rabbis also derive it from here. Yes this is so[, everyone actually does derive the principle of separation from here]. So rather then why do the Rabbis need the verse of kares b'achoso for [if they aren't learning the principle of separation]? To make one liable [to three korban chatoses if he inadvertently cohabited in the same of lapse of unawareness] with his sister, his father's sister, and his mother's sister.
פשיטא הרי גופין מוחלקין הרי שמות מוחלקין אלא לחייבו על אחותו שהיא אחות אביו שהיא אחות אמו והיכי משכחת לה ברשיעא בר רשיעא
This should be obvious!? They are separate bodies and they are separate prohibitions, [why do the Rabbis need a verse to teach us this?] Rather to make one liable [separate offerings for cohabiting with] his sister who is his father's sister, and who is [also] his mother's sister. And how can we find such a case, with a sinner the son of a sinner [re'shia bar re'shia. cf. Rashi].
ור' יצחק הא מנא ליה נפקא ליה מק"ו דתניא אמר ר"ע שאלתי את רבן גמליאל ורבי יהושע באיטליז של עימאום שהלכו ליקח בהמה למשתה בנו של ר"ג הבא על אחותו שהיא אחות אביו שהיא אחות אמו מהו [אינו] חייב על כולן אלא אחת או חייב על כל אחת ואחת
And from where does Rebbi Yitzchak learn [re'shia bar re'shia]? He learns it out from a kal v'chomer (fortiori), That it was taught in a barasia, Rabbi Akiva says, I asked Raban Gmaliel ad Rebbi Yehosua [while] in the meat market of Iemus, (that they had traveled to to purchase an animal for the wedding feast of Rabban Gamliel's son), If one had [inadvertently] cohabited with his sister, who was his father's sister, who his mother's sister, what would the halacha be? Would he only be liable one sacrifice for all of them, or would he be liable [a separate sacrifice] for each one?
אמרו לו זו לא שמענו אבל שמענו הבא על חמש נשים נדות בהעלם אחד שחייב על כל אחת ואחת ונראין דברים מק"ו ומה נדה שהיא שם אחד חייב על כל אחת ואחת כאן ששלשה שמות לא כל שכן
They reponded, This [case] we have not heard, but we have not heard, but we have heard that one who cohabits with five woman who are niddahs in one labse of unawareness is liable [a separate offering] for each and every one, and it would appear that these [two] cases form a kal v'chomer, and just if [by the case of five] niddah[s] which is one prohibition, one is liable for each and every one, then here [by re'shia bar re'shia] where there are three separate prohibitions is it not more so?
ולאידך נמי האי ודאי ק"ו פריכא הוא אלא נפקא ליה (ויקרא כ, יז) מאחותו דסיפא
And according to [Rebbi Yitzchak] also it is definitly a flawed kal v'chomer, rather then [Rebbi Yitzchak] learns out [re'shia bar re'shia] from achoso d'seifa [the extra repeated word 'sister' at the end of the verse].
ואידך אחותו דסיפא למה לי לחייבו על אחותו בת אביו ובת אמו לומר שאין עונשין מן הדין
And why do the Rabbis need achoso d'seifa? To make one liable for his sister who is his father's daughter and his mother's daughter [his full sister], to teach that we can't derive punishment from a kal v'chomer [specifically that we don't derive it from the prohibitions on both half sisters].
ואידך איבעית אימא גמר עונש מאזהרה ואיבעית אימא נפקא ליה
And [as for where Rebbi Yitzchak derives the principle against deriving punishments] you can either say that he derives [the principle against deriving] punishment from [the principle against deriving] scriptural warnings, or you can say that he learns it out from