Mishnayos Yevamos Perek 3 Mishnah 7

In the case of three brothers, two of whom were married to two sisters and one who was married to an unrelated woman, the following occurred: Shimon, the husband of one of the sisters, divorced his wife, and then Levi, who was married to the unrelated woman, died, and Shimon, the man who divorced his wife, married, i.e., performed levirate marriage with, her, i.e., this unrelated woman. And then Shimon himself later died, so that the unrelated woman happened for levirate marriage before Reuven, the third brother, who is married to the second sister. In this scenario, Reuven is allowed to consummate the levirate marriage with the unrelated woman. This is the case that was referred to when they said: And with regard to all those fifteen forbidden relatives who died or were divorced, their rival wives are permitted to enter into levirate marriage. This is because at the time that they happened before the yavam for levirate marriage they were no longer the rival wives of a forbidden relative.

שְׁלֹשָׁה אַחִים, שְׁנַיִם מֵהֶן נְשׂוּאִים שְׁתֵּי אֲחָיוֹת, וְאֶחָד נָשׂוּי נָכְרִית, מֵת אֶחָד מִבַּעֲלֵי אֲחָיוֹת, וְכָנַס נָשׂוּי נָכְרִית אֶת אִשְׁתּוֹ, וּמֵתָה אִשְׁתּוֹ שֶׁל שֵׁנִי, וְאַחַר כָּךְ מֵת נָשׂוּי נָכְרִית, הֲרֵי זוֹ אֲסוּרָה עָלָיו עוֹלָמִית, הוֹאִיל וְנֶאֶסְרָה עָלָיו שָׁעָה אֶחָת. שְׁלֹשָׁה אַחִים, שְׁנַיִם מֵהֶן נְשׂוּאִין שְׁתֵּי אֲחָיוֹת, וְאֶחָד נָשׂוּי נָכְרִית, גֵּרֵשׁ אֶחָד מִבַּעֲלֵי אֲחָיוֹת אֶת אִשְׁתּוֹ, וּמֵת נָשׂוּי נָכְרִית, וּכְנָסָהּ הַמְּגָרֵשׁ, וָמֵת, זוֹ הִיא שֶׁאָמְרוּ, וְכֻלָּן שֶׁמֵּתוּ אוֹ נִתְגָּרְשׁוּ, צָרוֹתֵיהֶן מֻתָּרוֹת: