יורה דעה רס״טYoreh De'ah 269

Se’if 1א׳

דִּין תּוֹרָה שֶׁמֻּתָּר לְגֵר שֶׁיִּשָּׂא אִמּוֹ, אוֹ אֲחוֹתוֹ מֵאִמּוֹ, שֶׁנִּתְגַּיְּרוּ. אֲבָל חֲכָמִים אָסְרוּ דָּבָר זֶה, כְּדֵי שֶׁלֹּא יֹאמְרוּ: בָּאנוּ מִקְּדֻשָּׁה חֲמוּרָה לִקְדֻשָּׁה קַלָּה. וְכֵן גֵּר שֶׁבָּא עַל אִמּוֹ אוֹ אֲחוֹתוֹ, וְהִיא בְּגֵיוּתָהּ, הֲרֵי זֶה כְּבָא עַל אֲרַמִּית.

It is biblical law that it is permitted for a convert to marry his mother, or his sister from his mother that converted. Yet the sages prohibited this so that they wouldn't say: we came from stricter holiness to lenient holiness. So too a convert who slept with his mother or sister and she is still a gentile, it is as if he slept with an Aramean.

Se’if 2ב׳

כֵּיצַד דִּין גֵּרִים בַּעֲרָיוֹת שֶׁל שְׁאֵר בָּשָׂר, אִם הָיָה נָשׂוּי כְּשֶׁהוּא עוֹבֵד כּוֹכָבִים לְאִמּוֹ אוֹ לַאֲחוֹתוֹ מֵאִמּוֹ, וְנִתְגַּיְּרוּ, מַפְרִישִׁין אוֹתָם. וְאִם הָיָה נָשׁוּי לִשְׁאָר עֲרָיוֹת, וְנִתְגַּיֵּר הוּא וְאִשְׁתּוֹ, אֵין מַפְרִישִׁין אוֹתָם.

What is the law of converts concerning the forbidden unions of near kin? If, when he was an idolater, he was married to his mother or to his sister from his mother, and they converted, they are separated. And if he was married to one of the other forbidden unions, and he and his wife converted, they are not separated.

Se’if 3ג׳

עוֹבֵד כּוֹכָבִים אָסוּר בִּשְׁאֵר הָאֵם אַחַר שֶׁנִּתְגַּיֵּר, מִדִּבְרֵי סוֹפְרִים, וּמֻתָּר בִּשְׁאֵר הָאָב, אַף עַל פִּי שֶׁיּוֹדֵעַ בְּוַדַּאי שֶׁזֶּה שְׁאֵרוֹ מֵאָבִיו, כְּגוֹן תְּאוֹמִים, שֶׁדָּבָר בָּרוּר הוּא שֶׁאָבִיו שֶׁל זֶה הוּא אָבִיו שֶׁל זֶה, אַף עַל פִּי כֵן לֹא גָּזְרוּ עַל שְׁאֵר אָבִיו. לְפִיכָךְ נוֹשֵׂא הַגֵּר אֵשֶׁת אָחִיו מֵאָבִיו, וְאֵשֶׁת אֲחִי אָבִיו, וְאֵשֶׁת אָבִיו, וְאֵשֶׁת בְּנוֹ, אַף עַל פִּי שֶׁנִּשֵּׂאת לְאָחִיו אוֹ לַאֲחִי אָבִיו אוֹ לִבְנוֹ אַחַר שֶׁנִּתְגַּיְּרוּ. וְכֵן אֲחוֹת אִמּוֹ מֵאָבִיהָ, וַאֲחוֹתוֹ מֵאָבִיו, וּבִתּוֹ, שֶׁנִּתְגַּיְּרוּ, מֻתָּרוֹת לוֹ. וְיֵשׁ אוֹסְרִים בְּאֵשֶׁת אָבִיו (טוּר בה"ג). אֲבָל אֵינוֹ נוֹשֵׂא לֹא אֲחוֹתוֹ מֵאִמּוֹ, וְלֹא אֲחוֹת אִמּוֹ מֵאִמָּהּ, וְלֹא אֵשֶׁת אָחִיו מֵאִמּוֹ, שֶׁנְּשָׂאָהּ אָחִיו מֵאִמּוֹ אַחַר שֶׁנִּתְגַיְּרָה, אֲבָל אִם נְשָׂאָהּ אָחִיו כְּשֶׁהָיָה עוֹבֵד כּוֹכָבִים, הֲרֵי זֶה מֻתֶּרֶת לוֹ.

An idolater is forbidden the kin of his mother after he has converted, by the words of the Scribes; and he is permitted the kin of his father, even though he knows for certain that this is his kin from his father — such as twins, where it is a clear matter that the father of this one is the father of that one; even so, they did not decree concerning the kin of his father. Therefore the convert marries the wife of his brother from his father, and the wife of his father's brother, and the wife of his father, and the wife of his son, even though she was married to his brother, or to his father's brother, or to his son after they converted. And likewise the sister of his mother from her father, and his sister from his father, and his daughter, who converted, are permitted to him. And there are those who forbid the wife of his father (Tur, the Baal Halachot Gedolot). But he does not marry his sister from his mother, nor his mother's sister from her mother, nor the wife of his brother from his mother whom his brother from his mother married after she converted; but if his brother married her when he was an idolater, behold, she is permitted to him.