Mishnayos Yevamos Perek 11 Mishnah 2

With regard to a female convert whose sons converted with her, they do not perform ḥalitza for each other’s wives, and they do not perform levirate marriage with them, as their conversions are considered rebirth, and they are considered unrelated. This is so even if the conception of the first son was not in the sanctity of Israel, i.e., the mother had not yet converted when she conceived of him, but his birth was in the sanctity of Israel, as his mother had converted by the time she gave birth to him, whereas the second son was both conceived and born in sanctity. The first son is considered a convert, who is unrelated to his brother. And this halakha similarly applies to a maidservant whose sons were freed with her, as they too are not considered relatives.

הַגִּיּוֹרֶת שֶׁנִּתְגַּיְּרוּ בָנֶיהָ עִמָּהּ, לֹא חוֹלְצִין וְלֹא מְיַבְּמִין, אֲפִלּוּ הוֹרָתוֹ שֶׁל רִאשׁוֹן שֶׁלֹּא בִקְדֻשָּׁה וְלֵדָתוֹ בִקְדֻשָּׁה, וְהַשֵּׁנִי הוֹרָתוֹ וְלֵדָתוֹ בִקְדֻשָּׁה. וְכֵן שִׁפְחָה שֶׁנִּשְׁתַּחְרְרוּ בָנֶיהָ עִמָּהּ: