Mishnayos Yevamos Perek 3 Mishnah 9

In the case of three brothers who were married to three unrelated women, and one of the brothers died, the following occurred: The second brother performed levirate betrothal with the wife of the deceased brother and before he was able to consummate the levirate marriage he died as well, leaving behind two women who happen before the third brother for levirate marriage. Then those two women must perform ḥalitza and may not enter into levirate marriage. As it is stated: “If brothers dwell together and one of them dies and he has no child, the wife of the dead man shall not be married outside of the family to one not of his kin; her brother-in-law will have intercourse with her” (Deuteronomy 25:5). This teaches that a woman eligible for levirate marriage is one who has one levirate relationship and not one who has a double levirate relationship. In this case, the wife of the first deceased brother requires levirate marriage due to both the marriage with her first husband as well as the levirate betrothal with the second brother. Rabbi Shimon says: He may consummate the levirate marriage with whichever woman he wishes and then perform ḥalitza with the second. In the case where two brothers were married to two sisters, and one of the brothers died, the widow at this point would be exempt from levirate marriage as the sister of his wife. And afterward the wife of the second brother died. Although the yevama is no longer the sister of his wife, this woman is nevertheless forbidden to him forever, since she had already been forbidden to him at one time.

שְׁלֹשָׁה אַחִין נְשׂוּאִין שָׁלֹשׁ נָכְרִיוֹת, וּמֵת אַחַד מֵהֶן, וְעָשָׂה בָהּ הַשֵּׁנִי מַאֲמָר, וָמֵת, הֲרֵי אֵלּוּ חוֹלְצוֹת וְלֹא מִתְיַבְּמוֹת, שֶׁנֶאֱמַר (דברים כה), וּמֵת אַחַד מֵהֶם יְבָמָהּ יָבֹא עָלֶיהָ, שֶׁעָלֶיהָ זִקַּת יָבָם אֶחָד, וְלֹא שֶׁעָלֶיהָ זִקַּת שְׁנֵי יְבָמִין. רַבִּי שִׁמְעוֹן אוֹמֵר, מְיַבֵּם לְאֵיזוֹ שֶׁיִּרְצֶה, וְחוֹלֵץ לַשְּׁנִיָּה. שְׁנֵי אַחִין נְשׂוּאִין לִשְׁתֵּי אֲחָיוֹת, וּמֵת אַחַד מֵהֶן, וְאַחַר כָּךְ מֵתָה אִשְׁתּוֹ שֶׁל שֵׁנִי, הֲרֵי זוֹ אֲסוּרָה עָלָיו עוֹלָמִית, הוֹאִיל וְנֶאֶסְרָה עָלָיו שָׁעָה אֶחָת: