Jerusalem Talmud Yevamot 1:5

משנה: שֵׁשׁ עֲרָיוֹת חֲמוּרוֹת מֵאֵילּוּ מִפְּנֵי שֶׁנְּשׂוּאוֹת לָאֲחֵרִים צָרוֹתֵיהֶן מוּתָּרוֹת. אִמּוֹ וְאֵשֶׁת אָבִיו וָאֲחוֹת אָבִיו אֲחוֹתוֹ מֵאָבִיו. וְאֵשֶׁת אֲחִי אָבִיו וְאֵשֶׁת אָחִיו מֵאָבִיו. MISHNAH: Six incest prohibitions are more stringent than these¹⁷¹Than the 15 mentioned in Mishnah 1. since they [concern women who] must be married to others¹⁷²They are equally forbidden to all paternal brothers and never can be parties to levirate., their co-wives are permitted¹⁷³If they were widowed or divorced and married an unrelated husband, their co-wives from this marriage are unrelated to the brothers since co-wives can become forbidden only in cases of levirate.: A man’s mother, his father’s wife, his father’s sister, his paternal sister, his paternal aunt, and his paternal brother’s wife¹⁷⁴Who had children or was divorced..

הלכה: שֵׁשׁ עֲרָיוֹת חֲמוּרוֹת מֵאֵילּוּ מִפְּנֵי שֶׁהֵן נְשׂוּאוֹת לָאֲחֵרִים כול׳. מַה אִינָּה חֲמוּרָה. הָכָא כָרֵת וְהָכָא כָרֵת. הָכָא מַמְזֵר וְהָכָא מַמְזֵר. מַהוּ חֲמוּרוֹת. שֶׁאֵינָן יְכוֹלוֹת לְהִינָּשֶׂא לָאַחִים שֶׁלֹּא בַעֲבֵירָה. וְתַנֵּי רִבִּי חִייָה כֵן. וְכוּלָּן אִם יְכוֹלוֹת לְהִינָּשֶׂא לָאַחִין שֶׁלֹּא בַעֲבֵירָה צָרוֹתֵיהֶן פְּטוּרוֹת מִן הַחֲלִיצָה וּמַן הַיִּבּוּם. HALAKHAH: “Six incest prohibitions are more stringent than these since they [concern women who] must be married to others”, etc. In what are they more stringent? Here there is extirpation, there is extirpation¹⁷⁵Incest with some of the women is a capital crime. Even these are subject to divine extirpation if there are no two witnesses to the act.. Here there is bastardy, there there is bastardy¹⁷⁶The child of any of the forbidden unions is a bastard excluded from marriage with natural-born Jews.. What is more stringent? That they cannot be married to the brothers without transgression. And Rebbi Ḥiyya stated thus: For all women who may be married to the brothers without transgression, their co-wives are free from ḥalîṣah and levirate¹⁷⁷A slightly different version in Tosephta 1:7..

תַּנֵּי רִבִּי חִייָה. אֵין צָרָה אֶלָּא מֵאָח בִּלְבַד. אָמַר רִבִּי יוֹסֵי. מַתְנִיתָא אָֽמְרָה כֵן. מִפְּנֵי שֶׁהֵן נְשׂוּאוֹת לָאֲחֵרִים צָרוֹתֵיהֶן מוּתָּרוֹת. רִבִּי יִרְמְיָה בָּעֵי. וּתְהֵא צָרָה מִן הַשּׁוּק. אָמַר רִבִּי יוֹסֵה. לֹא שְׁמִיעַ רִבִּי יִרְמְיָה הָדָא דְתַנֵּי רִבִּי חִייָה. לֹא שְׁמִיעַ לְמַתְנִיתִין מִפְּנֵי שֶׁהֵן נְשׂוּאוֹת לַאֲחֵרִים צָרוֹתֵיהֶן מוּתָּרוֹת. חָזַר וָמַר. אִין דִּשְׁמִיעַ. אֶלָּא כְּאִינַשׁ דִשְׁמִיעַ מִילָּה וּמַקְשֵׁי עֲלָהּ. Rebbi Ḥiyya stated¹⁷⁷A slightly different version in Tosephta 1:7.: “A co-wife is only from a brother”. Rebbi Yose said, our Mishnah says the same: “Since they must be married to others¹⁷⁸Others than the brothers., their co-wives are permitted.” Rebbi Jeremiah asked: Why can there not be a co-wife from the market place¹⁷⁹If an incest-prohibited woman is married to a total stranger, why should her co-wife, if divorced or widowed, not be forbidden also to the family of the other wife?? Rebbi Yose said, did Rebbi Jeremiah not hear what Rebbi Ḥiyya said, did he not hear our Mishnah, “since they must be married to others, their co-wives are permitted”¹⁸⁰It is impossible to become an ordained rabbi without knowing the Mishnah.? He turned around and said, certainly did he hear. But he is a person who hears something and questions it¹⁸¹The Yerushalmi leaves this without an answer. Two different answers are given in Sifra Qedošim Pereq11(12), Babli Yebamot 8a/b..

רִבִּי יִרְמְיָה בָּעֵי. הַבָּא עַל הַצָּרָה מַהוּ שֶׁיְּהֵא חַייָב עָלֶיהָ מִשּׁוּם אֵשֶׁת אַח. אָמַר רִבִּי יוֹסֵי. אִילּוּ אֶיפְשַׁר לְצָרָה בְּלֹא אֵשֶׁת אַח יְאוּת. אֶיפְשַׁר לְאֵשֶׁת אַח בְּלֹא צָרָה. אֵיפְשַׁר לְצָרָה בְּלֹא אֵשֶׁת אַח. Rebbi Jeremiah asked: If one has intercourse with a co-wife¹⁸²Widow of his brother whom he cannot marry because of the incest prohibition regarding the other wife., is he guilty because of his brother’s wife¹⁸³In general, the brother’s wife is forbidden under penalty of divine extirpation. But if levirate is possible, the widows of the dead brother become potentially permitted. If one of them is married by a brother, the others are forbidden since Deut. 25:5–9 speaks only of one sister-in-law and one house to build. The question is, does the brother who sleeps with the excluded co-wife infringe on the prescription (regarding a single levirate) of Deut. 25 or the incest prohibition of Lev. 18? The problem does not appear in the Babli since it derives all prohibitions directly from Lev. 18.? Rebbi Yose said, if a co-wife were possible without a brother’s wife, that would be a valid question¹⁸⁴In the sense of R. Jeremiah’s first question.. It is possible to be a brother’s wife without a co-wife. It is impossible to be a co-wife without a brother’s wife¹⁸⁵As R. Ḥiyya stated. Therefore, the prohibition of the co-wife is no separate prohibition, but only that one of the dead brother’s wives gets a special dispensation from the incest prohibition of Lev. 18:16..

וְאֵשֶׁת אָח לֹא יְבִמְתּוֹ הִיא. אָמַר רִבִּי יוֹסֵה בֵּירִבִּי בּוּן. תִּיפְתָּר שֶׁהָיָה לָהּ בָּנִים מֵאַחִים. Is the brother’s wife not his sister-in-law? Rebbi Yose bar Abun said, explain it if she had children from a brother¹⁷⁴Who had children or was divorced..