Sifrei Devarim 288

כי ישבו אחים. פרט לאחיו שלא היה בעולמו. מכאן אמרו, ב' אחים ומת אחד מהם ונולד להם אח, ואחר כך ייבם השני את אשת אחיו [ומת. הראשונה יוצאת משום אשת אחיו שלא היה בעולמו, והשניה משום צרתה].

(Devarim 25:5) "When brothers dwell together and one of them dies, and he has no son, the wife of the dead one shall not be outside to a strange man. Her levir (her husband's brother) shall come upon her and he shall take her for himself as a wife, and he shall have her in levirate marriage (yibum)."

יחדו. [המיוחדים בנחלה,] פרט לאחים מן האם. לפי שמצינו (שיש באחים מן התורה) [בעריות], ששוה אח מן האם לאח מן האב. יכול אף כאן כן? ת"ל "יחדיו", פרט לאחיו מאמו.

"When brothers dwell together": to exclude a brother who was not in his world (i.e., who was not concurrently alive). From here they ruled: If there were two brothers and one of them died and another one was born, after which the second brother performed yibum with the wife of the first, after which he died, then the first wife (i.e., the wife of the brother who died first) is exempt (from yibum) by reason of her being "the wife of his brother who was not in his world," and the second wife (i.e., the wife of the second brother) by reason of her being her (the first wife's) tzarah (co-wife). "together": in inheritance — to exclude maternal brothers. Because we find in arayoth (illicit relations) that a maternal brother is equated with a paternal brother, I might think that here, too, this is so. It is, therefore, written "together" (in inheritance), to exclude his maternal brother.

ומת אחד מהם. אין לי אלא בזמן שהם שנים ומת אחד מהם. מנין אפילו הם מרובין (? ת"ל ומת אחד מהם. מנין אפילו מתו כולם? ת"ל ומת אחד מהם, ולא נאמר "אחד מהם"). ומתו? ת"ל ומת מהם. א"כ מה ת"ל "אחד"? אשת אחד מתיבמת, ולא אשת שנים. מכאן אמרו, שלשה אחים נשואים ג' נכריות [ומת א' מהם, ועשה בה השני מאמר ומת - חולצת ולא מתיבמת. שנ' ומת אחד מהם, שעליה זיקת יבם אחד, ולא שעליה זיקת שני יבמין].

"and one of them dies": This tells me (that yibum obtains) only where there are two brothers and one of them dies. Whence do I derive (that yibum obtains) even if they are many and they die (that yibum obtains with the remaining brothers)? From "and if there die … of them." If so, why is it written "one"? Yibum obtains with the wife of one of them and not with the "wife" of two (see what follows). From here they ruled: If three brothers were married to three unrelated women, and one of them died, and the second made a ma'amar (betrothal of his yevamah by writ or money — short of cohabitation), and he, too, died, then she (the yevamah of the first,) receives chalitzah (yibum release), but not yibum, it being written "and one of them dies" — She who is linked to only one yavam (undergoes yibum), but not she who is linked to two, (as in the above instance, where the woman is linked to the remaining brother by the yibum tie and to the ma'amar brother by the ma'amar tie.)

לא תהיה אשת המת החוצה לאיש זר. למה אני צריך? לפי שאמרנו להלן, "אשת אחד מתיבמת ולא אשת שנים" (יכול אף כאן כן) [יכול תהא פטורה]? ת"ל לא תהי' אשת המת החוצה לאיש זר. כיצד הוא עושה? (או חולצת או מתייבמת) [חולצת ולא מתיבמת]. הנותן גט - פסלה לעצמו ופסלה ע"י אחים. יכול (יהיה גט פוטרה) [תהא פטורה] (בגט זה מזיקת האחים)? ודין הוא - ומה חליצה שאינה פוטרת באשה, פוטרת ביבם; גט שפוטר באשה, אינו דין שפוטר ביבם! ת"ל לא תהיה אשת המת החוצה לאיש זר, [אלא] בחליצה. העושה מאמר ביבמתו - קנאה לעצמו ופסלה על האחים. יכול יהיה מאמר גומר בה? ת"ל יבמה יבא עליה, ביאה גומרת בה ואין מאמר גומר בה.

"the wife of the dead one shall not be outside to a strange man": What is the intent of this? Because it was ruled: Yibum obtains with the wife of one but not with the "wife" of two (see above), I might think that she (the woman in the above instance) is (entirely) exempt. It is, therefore, written "the wife of the dead one shall not be outside to a strange man." What is to be done? She undergoes chalitzah (yibum release) but not yibum. If one gives a get (to his yevamah), he forbids her to himself and to his brothers. I might think that she is (entirely) exempt, and that this follows, viz.: If chalitzah, which does not exempt a woman (from the marriage tie) exempts her from the yibum tie, then get, which does exempt a woman (from the marriage tie), how much more so should it exempt her from the yibum tie! It is, therefore, written "the wife of the dead one shall not be outside to a strange man" — except by chalitzah. If one makes a ma'amar in his yevamah, he acquires her for himself and forbids her to the brothers. I might think that the ma'amar consummates the union. It is, therefore, written "her yavam shall come upon her" — Cohabitation consummates, and not ma'amar.

יבמה יבוא עליה. בין בשוגג בין במזיד בין באונס בין ברצון. ואפילו היא שוגגת והוא מזיד, או שהוא שוגג והיא מזידה.

"her yavam shall come upon her": whether unwittingly, (thinking her to be a different woman), wantonly, (intending illicit intercourse), constrainedly, or wittingly — even if she is unwitting and he wanton or he unwitting and she wanton, (he acquires her as a wife).

ולקח ולקחה ויבם ויבמה. פרט לצרות (מכל העריות וכן) [ועריות]. מכאן אמרו, ט"ו נשים פוטרות צרותיהן וצרות צרותיהן וכו'.

(Instead bf being written) "and he shall take" (it is written) "and he shall take her"; (instead of) "and he shall perform yibum," (it is written) "and he shall perform yibum (with) her" — to exclude (from yibum) co-wives (tzaroth) and arayoth (illicit relations) — whence it was ruled: Fifteen women exempt (from yibum) their co-wives and the co-wives of their co-wives, etc.