Yevamot — Daf 29a

והא א"ר שמעון שתי אחיות לא חולצות ולא מתייבמות

גזירה משום איסור מצוה דעלמא

הא תינח איהי אחותה מאי איכא למימר גזירה אחותה משום לתא דידה

והא גבי ערוה לא גזרינן שאני ערוה דמגמר גמירי לה אינשי וקלא אית לה:

מתני׳ שלשה אחין שנים נשואים שתי אחיות ואחד מופנה מת אחד מבעלי אחיות ועשה בה מופנה מאמר ואח"כ מת אחיו השני

בש"א אשתו עמו והלזו תצא משום אחות אשה וב"ה אומרים מוציא את אשתו בגט ובחליצה ואשת אחיו בחליצה זו היא שאמרו אוי לו על אשתו ואוי לו על אשת אחיו:

גמ׳ זו היא למעוטי מאי למעוטי הך דר' יהושע דלא עבדינן כוותיה אלא אי כר"ג אי כר"א

א"ר אלעזר לא תימא מאמר לב"ש קונה קנין גמור דאי בעי לאפוקי סגי לה בגיטא אלא מאמר לב"ש אינו קונה אלא לדחות בצרה בלבד

אמר רבי אבין אף אנן נמי תנינא בש"א יקיימו יקיימו אין לכתחלה לא

But did not R. Simeon state that two sisters  are neither to perform the halizah nor to be taken in levirate marriage!  — This  is a preventive measure against any other case where the prohibition is due to a commandment — 4 This is a satisfactory explanation in respect of herself;  what, however, can be said in respect of her sister?  -The provision was made in the case of her sister as a preventive measure against herself.  But, surely. no such preventive measures were made in the case where one was forbidden as incest!  — A case of incest is different because people are well acquainted with it  and it  is well known. MISHNAH. IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS AND THE THIRD WAS UNMARRIED,  AND WHEN ONE OF THE SISTERS HUSBANDS DIED, THE UNMARRIED BROTHER ADDRESSED TO HER  A MA'AMAR,  AND THEN HIS SECOND BROTHER DIED, BETH SHAMMAI SAY: HIS WIFE  [REMAINS] WITH HIM WHILE THE OTHER IS EXEMPT  AS BEING HIS WIFE'S SISTER.  BETH HILLEL, HOWEVER, MAINTAIN THAT HE MUST DISMISS HIS WIFE  BY A LETTER OF DIVORCE  AND BY HALIZAH,  AND HIS BROTHER'S WIFE BY HALIZAH.  THIS IS THE CASE IN REGARD TO WHICH IT WAS SAID: WOE TO HIM BECAUSE OF HIS WIFE, AND WOE TO HIM BECAUSE OF HIS BROTHER'S WIFE. GEMARA. What was THIS IS meant to exclude?  — To exclude the statement  of R. Joshua,  [and to indicate] that we do not act In accordance with his view but either in accordance with that of R. Gamaliel or that of R. Eliezer. R. Eleazar said: It must not be assumed that a ma'amar according to Beth Shammai constitutes a perfect kinyan,  so that, if he  wishes to dismiss her, a letter of divorce is sufficient; but rather that, according to Beth Shammai, a ma'amar constitutes a kinyan only so far as to keep out the rival.  Said R. Abin: We also have learned the same thing: Beth Shammai said, 'They may retain them',  which implies that they may only retain them  but [that they may] not [marry them] at the outset.