Mishnayos Yevamos Perek 11 Mishnah 4

With regard to a woman whose offspring was mixed with the offspring of her daughter-in-law, and their lineage was consequently indeterminate, and the mixed sons matured and married women, and subsequently they died, the certain sons of the daughter-in-law perform ḥalitza with the wives, but not levirate marriage, as with regard to each wife it is uncertain whether she is his brother’s wife, and therefore his yevama, and uncertain whether she is his father’s brother’s wife, who is forbidden to him. However, the certain sons of the elder woman, i.e., the mother-in-law, perform either ḥalitza or levirate marriage, as with regard to each wife it is uncertain whether she is his brother’s wife, in which case levirate marriage is valid, or his brother’s son’s wife, in which case she is permitted to him, after having performed ḥalitza with a son of the daughter-in-law. If the sons of certain, unflawed lineage were the ones who died, then the mixed sons perform ḥalitza with the widows of the elder woman’s sons but not levirate marriage, as it is uncertain whether she is his brother’s wife or his father’s brother’s wife. With the widows of the certain sons of the daughter-in-law, one of the mixed sons performs ḥalitza, in case she is his brother’s wife. And the other one performs levirate marriage, as even if she is his brother’s son’s wife, she is permitted to him.

הָאִשָּׁה שֶׁנִּתְעָרֵב וְלָדָהּ בִּוְלַד כַּלָּתָהּ, הִגְדִּילוּ הַתַּעֲרֹבוֹת, וְנָשְׂאוּ נָשִׁים, וּמֵתוּ, בְּנֵי הַכַּלָּה חוֹלְצִין וְלֹא מְיַבְּמִין, שֶׁהוּא סְפֵק אֵשֶׁת אָחִיו סְפֵק אֵשֶׁת אֲחִי אָבִיו. וּבְנֵי הַזְּקֵנָה, אוֹ חוֹלְצִין אוֹ מְיַבְּמִין, שֶׁהוּא סְפֵק אֵשֶׁת אָחִיו וְאֵשֶׁת בֶּן אָחִיו. מֵתוּ הַכְּשֵׁרִים, בְּנֵי הַתַּעֲרֹבוֹת לִבְנֵי הַזְּקֵנָה חוֹלְצִין וְלֹא מְיַבְּמִין, שֶׁהוּא סְפֵק אֵשֶׁת אָחִיו וְאֵשֶׁת אֲחִי אָבִיו, וּבְנֵי הַכַּלָּה, אֶחָד חוֹלֵץ וְאֶחָד מְיַבֵּם: