Rif Yevamot 30a {Yevamot 94a - 95a}

30a

{Yevamot 94a}
Gemara:

אמר רב הוה ליה לאלעזר בן מתיא למידרש בהאי קרא מרגניתא ודרש בה חספא
מאי מרגניתא ואשה גרושה מאישה אפי' לא נתגרשה אלא מאישה נפסלה מן הכהונה
כגון דאמר לה הרי את מגורשת ממני ואי את מותרת לכל אדם אע"ג דלעלמא לא הוו גרושין ולא שריא לאינסובי בהאי גיטא לדידיה מיהא איפסלא דאי כהן הוא איפסילא מיניה והיינו ריח הגט דפסיל בכהונה:

Rav {our gemara: Rav Yehuda cited Rav}: Eleazar ben Matia had a pearl to darshen in this verse and instead he darshened a shard of clay.
What is the pearl? {Vayikra 21:7}:

ז אִשָּׁה זֹנָה וַחֲלָלָה לֹא יִקָּחוּ, וְאִשָּׁה גְּרוּשָׁה מֵאִישָׁהּ לֹא יִקָּחוּ: כִּי-קָדֹשׁ הוּא, לֵאלֹהָיו.

7 They shall not take a woman that is a harlot, or profaned; neither shall they take a woman put away from her husband; for he is holy unto his God.

even if she was only divorced from her husband, she is invalid to marry a kohen. Such as where he said to her "behold you are divorced to me and you are not permitted to all men," although to the world it is not divorce and she is not permitted to marry via this get, to him however she is invalid, such that if he is a kohen, she is invalid to him, and this is the scent of a get which invalidates to marry a kohen.

Mishna:

מי שהלכה אשתו למדינת הים ובאו ואמרו לו מתה אשתו ונשא את אחותה ואחר כך באה אשתו מותרת לחזור לו
ומותר בקרובות שניה
ושניה מותרת בקרוביו
ואם מתה הראשונה מותר בשניה

Someone whose wife went to an overseas country and they said to him that his wife died, and he married her sister, and afterwards his wife came {back}, she is permitted to return to him
{Yevamot 94b}
and he is permitted in the relatives of the second one {= the sister}, and the second one is permitted to his relatives, and if the first one dies, he is permitted to the second one.

אמרו לו מתה אשתו ונשא אחותה ואח"כ אמרו לו קיימת היתה ומתה הולד הראשון ממזר והאחרון אינו ממזר
רבי יוסי אומר כל שהוא פוסל על ידי אחרים פוסל על ידי עצמו וכל שאינו פוסל על ידי אחרים אינו פוסל על ידי עצמו

If they told him that his wife died and he married her sister, and afterwards they told him that she had been actually alive and then she died, the first child {born in the interim, when the wife was alive} is a bastard, and the latter one {after the first wife's death} is not a bastard.
Rabbi Yossi says: Whoever invalidates for others invalidates for himself, and whoever does not invalidate others does not invalidate for himself.

Gemara:

ואף על גב דאזלא אשתו וגיסו למדינת הים דאהנו הני נשואין דקא מיתסרא אשת גיסו אגיסו אשת גיסו אסירא ואשתו שריא ולא אמרינן מתוך שנאסרה אשת גיסו אגיסו תיאסר אשתו עליו
לימא מתניתין דלא כרבי עקיבא דאי רבי עקיבא הויא לה אחות גרושתו
דתניא כל עריות שאמרו אין צריכות הימנו גט חוץ מאשת איש שנשאת על פי בית דין או ע"פ עדים
ורבי עקיבא מוסיף אף אשת אח ואחות אשה
כיון דאמר רבי עקיבא בעיא גט ממילא איתסרא עליה דהויא לה אחות גרושתו

And even though his wife and his brother-in-law {=the husband of his wife's sister} went to an overseas country, such that this "marriage" {to the truly married sister of his living wife} effectively prohibits the wife of his brother-in-law to his brother-in-law, it is the wife of his brother-in-law who is forbidden and his own wife is permitted, and we do not day that since the wife of his brother-in-law is forbidden on his brother in law, his own wife is forbidden upon him.

Let us say that our Mishna is not in accordance with Rabbi Akiva, for if it is like Rabbi Akiva, she {=his wife} would be the sister of his divorcee. For they learnt {in a brayta}: All the incestuous marriages they spoke of, they do not require of ihm a get except for a married woman who married {another} on the basis of Bet Din or on the basis of witnesses. And Rabbi Akiva adds: Also the wife of a brother and the sister of a wife.

Since Rabbi Akiva said that she requires a get, perforce she {his wife} is forbidden upon him for she is the sister of his divorcee!

ולאו מי איתמר עלה
אמר רב גידל אמר רב חייא בר יוסף אמר רב האי אשת אח היכי דמי
כגון שקידש אחיו את האשה והלך למדינת הים ושמע שמת אחיו ועמד ויבם את אשתו ואחר כך בא אחיו דאמרי אינשי הך קמא תנאה הוה ליה בקידושין והאי שפיר קא נסיב
והאי אחות אשה נמי היכי דמי כגון שקידש אשה והלכה לה למדינת הים ושמע שמתה ועמד ונשא את אחותה דאמרי אינשי הך קמייתא תנאה הוה ליה בקידושין והאי שפיר אבל בנשואין דליכא למימר תנאה הוה ליה בנשואין לא אמר רבי עקיבא דצריכה הימנו גט הילכך הויא מתניתין כוותיה
וכיון דמתניתין כוותיה הלכתא כוותיה

But is there not one who states regarding it as follows? That
Rav Gidel cited Rav Chiyya bar Yosef who cited Rav: This wife of a brother, how so? Such as where his brother betrothed a woman and then went to an overseas country, and then he heard that his brother died, and he stood and performed yibbum on his {brother's} wife, and afterwards his brother came -- for people might say that this first one {=the brother} had a condition in the betrothal, and this {other} one accomplished a fine marriage.
And this sister of a wife also, how so? Such as where he betrothed a woman and she went to an overseas country, and he heard that she died, and he stood and married her sister -- that people will say that with the first one made a condition in his betrothal, and {with} this one he married finely.
But by full marriage {nisuin}, where one cannot say that he made a condition in his nisuin, Rabbi Akiva will not say that she requires of him a get. Therefore our Mishna is in accordance with him.
And since our Mishna is in accordance with him, the halacha is like him.

וחזינן מאן דכתב כל האסורות לאדם איסור ערוה ובא עליהן אין צריכות הימנו גט חוץ מאחות אשתו ואשת אחיו ואשת איש שנשאת על פי בית דין מפני שיש להן היתר אחר איסורן גזירה שמא יאמרו בהיתר נשאת ויוצאת בלא גט ולא קא מפליג בין נשואין לקדושין ואנן כתבינן מאי דסבירא לן:

And we have seen one who wrote that all who are forbidden to a man as a prohibition of incest {erva} and he had intercourse with them, they do not require from him a get, except for the sister of his wife and the wife of his brother, and a married woman who remarried on the say-so of Bet Din, because they had to them a permission after their initial prohibition -- as a decree lest they {=people} say that she was married while permitted yet went out without a get, and he does not distinguish between nisuin and betrothal. And we have written what is logical to us {that is, not like that opinion}.

{Yevamot 95a}

תניא ושכב איש אותה אותה שכיבתה אוסרתה ואין שכיבת אחותה אוסרתה
והלא דין הוא ומה במקום שבא על איסור קל שהיא אשת איש שאין האוסרה אוסרה כל ימיו נאסר האוסר מקום שבא על איסור חמור שהיא אחות אשה שאחותה אוסרתה על בעלה כל ימי חייה אינו דין שנאסר האוסר
אמר רבי יהודה לא נחלקו בית שמאי ובית הלל בבא על חמותו שפסל את אשתו ועל מה נחלקו בבא על אחות אשתו
שבית שמאי אומרים פסל ובית הלל אומרים לא פסל

They learnt {in a brayta}: {Bemidbar 5:13}

יג וְשָׁכַב אִישׁ אֹתָהּ, שִׁכְבַת-זֶרַע, וְנֶעְלַם מֵעֵינֵי אִישָׁהּ, וְנִסְתְּרָה וְהִיא נִטְמָאָה; וְעֵד אֵין בָּהּ, וְהִוא לֹא נִתְפָּשָׂה.	13 and a man lie with her carnally, and it be hid from the eyes of her husband, she being defiled secretly, and there be no witness against her, neither she be taken in the act;
יד וְעָבַר עָלָיו רוּחַ-קִנְאָה וְקִנֵּא אֶת-אִשְׁתּוֹ, וְהִוא נִטְמָאָה; אוֹ-עָבַר עָלָיו רוּחַ-קִנְאָה וְקִנֵּא אֶת-אִשְׁתּוֹ, וְהִיא לֹא נִטְמָאָה.	14 and the spirit of jealousy come upon him, and he be jealous of his wife, and she be defiled; or if the spirit of jealousy come upon him, and he be jealous of his wife, and she be not defiled;

Her - her sleeping with forbids her, but sleeping with her sister does not forbid her.
And is it not logically inferable? If in the instance that he has intercourse with someone where the prohibition is light -- which is the wife of a{nother} man, that he who causes her to become forbidden does not cause her to be forbidden all his days -- he that caused the prohibition is forbidden {to her}, in an instance where he had intercourse with someone where the prohibition was heavy -- which is the sister of a wife, since her sister causes her to be forbidden to her husband all the days of her life -- is it not inferable that the one who caused the prohibition {=the wife who caused the prohibition to her sister} is forbidden?

Rabbi Yehuda said: Bet Shammai and Bet Hillel do not disagree about one who had intercourse with his mother-in-law that he invalidates his wife {from living with him}. And regarding what do they disagree? Regarding one who had intercourse with the sister of his wife. That Bet Shammai say he invalidates her, and Bet Hillel say he does not invalidate her.