Rif Yevamot 30b {Yevamot 95a continues; 95b}

30b

{Yevamot 95a continues}

א"ר יוסי לא נחלקו בית שמאי ובית הלל בבא על אחות אשתו שלא פסל
ועל מה נחלקו בבא על חמותו שבית שמאי אומרים פסל ובית הלל אומרים לא פסל

אמר רב יהודה אמר שמואל אין הלכה כר' יהודה
ההוא דעבד איסורא מחמותיה
אתייה רב יהודה ונגדיה
אמר ליה אי לאו דאמר שמואל אין הלכה כר' יהודה אסרתה לאיתתך עלך איסורא דלעלם:

Rabbi Yossi said: Bet Shammai and Bet Hillel do not disagree about one who had intercourse with his wife's sister that he does not invalidate {his wife}. And about what do they argue? About one who had intercourse with his mother-in-law. For Bet Shammai say he invalidates and Bet Hillel say he does not invalidate.

Rav Yehuda cited Shmuel: The halacha is not like Rabbi Yehuda.
There was a person who did a prohibited act with his mother-in-law. Rav Yehuda brought him before him and flogged him. He {Rav Yehuda} said to him: If not for the fact that Shmuel said that the halacha is not like Rabbi Yehuda, I would have forbidden your wife upon you forever.

{Yevamot 95b}

רבי יוסי אומר כל שפוסל על ידי אחרים פוסל ע"י עצמו וכו'
איבעיא לן מאי קאמר רבי יוסי
אמר ר' אמי רבי יוסי ארישא דמתני' קאי דקתני נשאת ע"פ [ב"ד] תצא ופטורה מן הקרבן [שלא] ע"פ ב"ד תצא וחייבת בקרבן ות"ק סבר לא שנא ע"פ ב"ד ולא שנא על פי עדים אשתו שריא ואמר ליה רבי יוסי על פי ב"ד דפסיל על ידי אחרים פוסל ע"י עצמו על פי עדים דאינו פוסל ע"י אחרים אינו פוסל על ידי עצמו

"Rabbi Yossi says: Whoever invalidates for others invalidates for himself ... {= and whoever does not invalidate others does not invalidate for himself}":
It was a question to us: What is Rabbi Yossi saying here?
Rabbi Ami said: Rabbi Yossi is going on the resha {first portion} of our Mishna, which taught "if she was married on the say-so of Bet Din, she goes out and is exempt from bringing a sacrifice; not on the say-so of Bet Din, she goes out and is obligated to bring a sacrifice." And the First Tanna holds that there is no difference between on the say-so of Bet Din or on the say-so of witnesses, she is permitted.
And Rabbi Yossi says to him that if it is on the say-so of Bet Din that "he invalidates for others " {since his wife's sister may not return to her husband} he invalidates for himself {such that his wife cannot return to him either}; if it is on the say-so of witnesses, where he does not invalidate for others {since his wife's sister can return to her husband}, he does not invalidate for himself {and his wife can return to him}.

ורבי יצחק נפחא אמר לעולם ר' יוסי אסיפא דמתני' פליג דשמעיה לתנא קמא דקאמר לא שנא דאזיל אשתו וגיסו למדינת הים ולא שנא דאזיל ארוסתו וגיסו למדינת הים ובאו ואמרו לו מתה אשתך ומת גיסך ונשא את אחותה ואחר כך באו אשתו וגיסו אשת גיסו אסורה אגיסו ואשתו מותרת לו
אמר ליה ר' יוסי ארוסתו וגיסו דאיכא למימר תנאה הוה ליה בקידושין והאי דנסיב לאחותה שפיר קא נסיב ובעיא גיטא מיניה דלא לימרו אשת איש יוצאה בלא גט דפסיל לה ע"י אחרים דהיינו גיסו דכיון דנפקא מיניה בגיטא איפסילא לה מבעלה הכי נמי פוסל ע"י עצמו ואיתסרא ליה איתתיה עליה דהויא לה אחות גרושתו
אבל היכא דאזול אשתו וגיסו למדינת הים וקא נסיב לה לאחותה דליכא למימר תנאה ה"ל בנשואין ושפיר קא נסיב לה להאי כדאיכא למימר באירוסין דלא בעיא גיטא מיניה ולא קא פסיל על ידי אחרים דהא לא איתסרא אשת גיסו אגיסו אינו פוסל נמי על ידי עצמו ושריא ליה איתתיה דהא ליתא אחות גרושתו

And Rabbi Yitzchak Nafcha said: In truth, Rabbi Yossi was going on the seifa {latter part} of our Mishna, and the statement is to be split. To the First Tanna who said that it does not matter whether the case is that his wife and brother-in-law {=the husband of his wife's sister} went to an overseas country or whether the case is that his betrothed and his brother-in-law went to an overseas country, and they came and told him "your wife died and your brother-in-law died," and he married her {=his wife's} sister, and afterwards his wife and his brother-in-law came, she {the wife's sister} is forbidden on his brother-in-law and his own wife is permitted to him. Rabbi Yossi said to him: In the case of his betrothed and his brother-in-law, it is possible to say that there was a condition in his betrothal and this that he married the sister was a fine marriage, and she requires from him a get, so that they should not say that a married woman went out without a get, thus he invalidates her for others -- which is to say for his brother-in-law, for since she went out from him with a get, she is invalidated from her husband -- so too he invalidates for himself, and his wife is forbidden to him, for she is {now} the sister of his divorcee.
But, if his wife {rather than betrothed} and his brother-in-law {=husband of his wife's sister} went to an overseas country, and he married the sister, where one cannot say that there was a condition in the nuptials, and {one cannot say} that he married her finely, as one could say by betrothal, such that she {the sister} does not require of him a get and he thus does not invalidate her for others, for the wife of his brother-in-law is not forbidden to the brother-in-law, he also does not invalidate for himself, and his wife is permitted to him, for she is not the sister of his divorcee.

אמר רב יהודה אמר שמואל הלכה כר' יוסי
ואיסתפק לן בגמרא דהא דאמר שמואל הלכה כר' יוסי אליבא דמאן אי אליבא דרבי אמי אי אליבא דר' יצחק נפחא
וכיון דלא איתברר כמאן מינייהו
פסק לא שרינא לאשת גיסו אגיסו מספיקא

Rav Yehuda cited Shmuel: The halacha is like Rabbi Yossi.
And it is a doubt to us in the gemara, this that Shmuel said that the halacha is like Rabbi Yossi, according to whom he was saying this -- if according to Rabbi Ami or according to Rabbi Yitzchak Nafcha.
And since it is not clarified according to which of them he rules so, we do not permit the wife of his brother-in-law because of doubt.