Mishnayos Yevamos Perek 2 Mishnah 2

If there were two brothers, and one died, and the second entered into levirate marriage with his brother’s wife while he was already married to another woman, and subsequently a third brother was born to them, and the second brother then died, whereby both of his wives happened before the third brother for levirate marriage, then the first woman, who was the wife of the first brother, is exempt due to the fact that she is the wife of a brother with whom the third brother did not coexist, and the second woman, who was the first wife of the second brother, is exempt due to her rival wife. If the second brother had performed only levirate betrothal with her and then died before fully marrying her, the second woman performs ḥalitza and may not enter into levirate marriage, as the levirate betrothal is not considered a sufficiently valid marriage so as to render her the rival wife of a relation forbidden to the third brother. Rabbi Shimon says with regard to the first clause of the mishna: The third brother either enters into levirate marriage with whichever one he wishes, or he performs ḥalitza with whichever one he wishes. Since he was born after his second brother had already entered into levirate marriage with the first brother’s widow, she is considered the wife of a brother with whom he did coexist, not the wife of a brother with whom he did not coexist. Therefore, he may enter into levirate marriage with her.

שְׁנֵי אַחִים וּמֵת אֶחָד מֵהֶן, וְיִבֵּם הַשֵּׁנִי אֶת אֵשֶׁת אָחִיו, וְאַחַר כָּךְ נוֹלַד לָהֶן אָח, וָמֵת, הָרִאשׁוֹנָה יוֹצֵאת מִשּׁוּם אֵשֶׁת אָחִיו שֶׁלֹּא הָיָה בְעוֹלָמוֹ, וְהַשְּׁנִיָּה מִשּׁוּם צָרָתָהּ. עָשָׂה בָהּ מַאֲמָר, וָמֵת, הַשְּׁנִיָּה חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת. רַבִּי שִׁמְעוֹן אוֹמֵר, מְיַבֵּם לְאֵיזוֹ מֵהֶן שֶׁיִּרְצֶה, אוֹ חוֹלֵץ לְאֵיזוֹ מֵהֶן שֶׁיִּרְצֶה: