Mishnayos Gittin Perek 8 Mishnah 6

The mishna teaches another halakha associated with the previous halakhot: With regard to all of those cases in which they said that a man who died without children and left behind a widow who is, to the man’s brother, one of those with whom relations are forbidden, e.g., she is his wife’s sister, not only is there no levirate bond for her, but the rival wives of the brother who died are also permitted to marry without either levirate marriage or ḥalitza. The mishna discusses another case: These rival wives went and married another man without ḥalitza, and these widows with whom relationships were forbidden were found to be sexually underdeveloped women incapable of bearing children [ailonit]. Therefore, it became clear, retroactively, that the marriage to the dead brother was never valid, and accordingly, the rival wives were never exempt from the obligation of levirate marriage due to their being the rival wives of a forbidden relationship. Consequently, the rival wives were forbidden to marry anyone else without ḥalitza, and the rival wives must leave both this man whom they remarried, and that yavam, i.e., they cannot enter into levirate marriage with him. And all of those above-mentioned ways of penalizing a woman who remarried based on the bills of divorce detailed in the earlier clause of the mishna apply to her in this case as well.

כָּל הָעֲרָיוֹת שֶׁאָמְרוּ צָרוֹתֵיהֶן מֻתָּרוֹת, הָלְכוּ הַצָּרוֹת הָאֵלּוּ וְנִשְּׂאוּ וְנִמְצְאוּ אֵלּוּ אַיְלוֹנִיּוֹת, תֵּצֵא מִזֶּה וּמִזֶּה, וְכָל הַדְּרָכִים הָאֵלּוּ בָהּ: