To not uncover the nakedness of his sister from the father, and she is the daughter of the wife of the father: To not uncover the nakedness of the daughter of the wife of the father when she is his sister — meaning to say, that she is the daughter of his father. As if she was the daughter of the wife of his father that is not his sister, we have already said above (Sefer HaChinukh 192) that he is permitted to marry her from the outset. Rather, the understanding [of the prohibition] is that it is the daughter of the wife of his father and she is his sister, as it is stated (Leviticus 18:11), “The nakedness of the daughter of your father [...] you shall not reveal.” And I have already written above regarding the nakedness of the sister that this negative commandment comes in order to make his sister from the wife of his father a sexual prohibition by itself — and similar to what I wrote about his mother (Sefer HaChinukh 190): that if she is the wife of the father, he is liable with her for two — on account of the mother and on account of the wife of the father. And the proof is from that which they, may their memory be blessed, said in Yevamot 22b, “The Sages learned, ‘He who has intercourse with his sister who is the daughter of his father’s wife, is liable on account of his sister and on account of the daughter of his father’s wife.’ Rabbi Yose beRebbi Yehudah says, ‘He is liable only on account of his sister alone.’ What is the reason for the opinion of the rabbis? They said, ‘How is it? Since it is written in the Torah, “The nakedness of your sister, the daughter of your father, etc.” (Leviticus 18:9), for what do I need, “The nakedness of the daughter of the wife of your father, who has born into your father’s household,” (Leviticus 18:11)? We understand from this, [that] it is to make him liable on account of his sister and on account of the daughter of his father’s wife.’” It comes out that there is only one negative commandment with one who lays with his sister who is from a woman that his father raped; but one who lays with his sister from a woman that his father married is liable for [the transgression of] two negative commandments — and [so] if inadvertent, he is obligated to bring two sin-offerings. And “the judgments of God are righteous”: Since the Torah distanced the sister, it is fitting to be stringent with the one that is his full sister — meaning to say from marriage, as she is called the true sister. And like this matter did Ramban, may his memory be blessed, write (Ramban on Leviticus 18:9).
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid volitionally with his sister from a woman that his father married is liable for excision; inadvertently, he is liable two sin-offerings, as we said.
שֶׁלֹּא לְגַלּוֹת עֶרְוַת אֲחוֹתוֹ מִן הָאָב וְהִיא בַּת אֵשֶׁת אָבִיו – שֶׁלֹּא לְגַלּוֹת עֶרְוַת בַּת אֵשֶׁת אָבִיו כְּשֶׁתִּהְיֶה אֲחוֹתוֹ. כְּלוֹמַר שֶׁהִיא בַּת אָבִיו, שֶׁאִלּוּ בַּת אֵשֶׁת אָבִיו שֶׁאֵינָהּ אֲחוֹתוֹ כְּבָר אָמַרְנוּ לְמַעְלָה (מצוה קצב) שֶׁמֻּתָּר לִשָּׂא אוֹתָהּ לְאִשָּׁה לְכַתְּחִלָּה, אֶלָּא פֵּרוּשׁוֹ, בַּת אֵשֶׁת אָבִיו וְהִיא אֲחוֹתוֹ, שֶׁנֶּאֱמַר (ויקרא יח יא) עֶרְוַת בַּת אֵשֶׁת אָבִיךָ לֹא תְגַלֵּה. וּכְבָר כָּתַבְתִּי לְמַעְלָה בְּעֶרְוַת אָחוֹת, שֶׁהַלָּאו הַזֶּה בָּא כְּדֵי שֶׁיָּשִׂים אֲחוֹתוֹ מִנְּשׂוּאַת אָבִיו עֶרְוָה בִּפְנֵי עַצְמָהּ, וּכְעֵין מַה שֶּׁאָמַרְנוּ בְּאִמּוֹ (מצוה קצ), שֶׁאִם הִיא אֵשֶׁת אָב חַיָּב בָּהּ שְׁתַּיִם מִשּׁוּם אֵם וּמִשּׁוּם אֵשֶׁת אָב. וְהָרְאָיָה מָה שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה בִּיבָמוֹת (כב, ב) תָּנוּ רַבָּנַן הַבָּא עַל אֲחוֹתוֹ, וְהִיא בַּת אֵשֶׁת אָבִיו, חַיָּב מִשּׁוּם אֲחוֹת וּמִשּׁוּם בַּת אֵשֶׁת אָבִיו. רַבִּי יוֹסֵי בְּרַבִּי יְהוּדָה אוֹמֵר אֵינוֹ חַיָּב אֶלָּא מִשּׁוּם אֲחוֹתוֹ בִּלְבַד. מַאי טַעְמַיְהוּ דְּרַבָּנָן אָמְרִי מִכְּדֵי כְּתִיב עֶרְוַת אֲחוֹתְךָ בַת אָבִיךָ וְגוֹ', עֶרְוַת בַּת אֵשֶׁת אָבִיךָ מוֹלֶדֶת אָבִיךָ לָמָּה לִי, שְׁמַע מִנַּהּ לְחַיְּבוֹ מִשּׁוּם אֲחוֹתוֹ וּמִשּׁוּם בַּת אֵשֶׁת אָבִיו. נִמְצָא שֶׁהַשּׁוֹכֵב עִם אֲחוֹתוֹ שֶׁהִיא מֵאֲנוּסַת אָבִיו אֵין בָּהּ אֶלָּא לָאו אֶחָד, וְהַשּׁוֹכֵב עִם אֲחוֹתוֹ שֶׁהִיא מִנְּשׂוּאַת אָבִיו יִתְחַיֵּב שְׁנֵי לָאוִין, וּבְשׁוֹגֵג חַיָּב לְהָבִיא שְׁתֵּי חַטָּאוֹת. וּמִשְׁפְּטֵי הַשֵּׁם צָדְקוּ, כִּי אַחַר שֶׁתַּרְחִיק הַתּוֹרָה קֻרְבַת הָאָחוֹת רָאוּי לְהַחְמִיר בְּאוֹתָהּ שֶׁהִיא אָחוֹת גְּמוּרָה, רוֹצֶה לוֹמַר מִן הַנִּשּׂוּאִין, כִּי הִיא נִקְרֵאת אֲחוֹת אֱמֶת. וּכְעִנְיָן זֶה כָּתַב הָרַמְבַּ"ן זִכְרוֹנוֹ לִבְרָכָה (אחרי מות יח ט).
To not uncover the nakedness of his sister from the father, and she is the daughter of the wife of the father: To not uncover the nakedness of the daughter of the wife of the father when she is his sister — meaning to say, that she is the daughter of his father. As if she was the daughter of the wife of his father that is not his sister, we have already said above (Sefer HaChinukh 192) that he is permitted to marry her from the outset. Rather, the understanding [of the prohibition] is that it is the daughter of the wife of his father and she is his sister, as it is stated (Leviticus 18:11), “The nakedness of the daughter of your father [...] you shall not reveal.” And I have already written above regarding the nakedness of the sister that this negative commandment comes in order to make his sister from the wife of his father a sexual prohibition by itself — and similar to what I wrote about his mother (Sefer HaChinukh 190): that if she is the wife of the father, he is liable with her for two — on account of the mother and on account of the wife of the father. And the proof is from that which they, may their memory be blessed, said in Yevamot 22b, “The Sages learned, ‘He who has intercourse with his sister who is the daughter of his father’s wife, is liable on account of his sister and on account of the daughter of his father’s wife.’ Rabbi Yose beRebbi Yehudah says, ‘He is liable only on account of his sister alone.’ What is the reason for the opinion of the rabbis? They said, ‘How is it? Since it is written in the Torah, “The nakedness of your sister, the daughter of your father, etc.” (Leviticus 18:9), for what do I need, “The nakedness of the daughter of the wife of your father, who has born into your father’s household,” (Leviticus 18:11)? We understand from this, [that] it is to make him liable on account of his sister and on account of the daughter of his father’s wife.’” It comes out that there is only one negative commandment with one who lays with his sister who is from a woman that his father raped; but one who lays with his sister from a woman that his father married is liable for [the transgression of] two negative commandments — and [so] if inadvertent, he is obligated to bring two sin-offerings. And “the judgments of God are righteous”: Since the Torah distanced the sister, it is fitting to be stringent with the one that is his full sister — meaning to say from marriage, as she is called the true sister. And like this matter did Ramban, may his memory be blessed, write (Ramban on Leviticus 18:9).
דִּינֵי הַמִּצְוָה. בְּסוֹף הַסֵּדֶר (מצוה ריא) תִּרְאֶה הֵיכָן הֵם [א"ה שם].
And at the end of the Order (Sefer HaChinukh 211), we will see where the laws of the commandments are. (See Tur, Even HaEzer 21.)
וְנוֹהֵג אִסּוּר זֶה בְּכָל מָקוֹם וּבְכָל זְמַן. וְהָעוֹבֵר עַל זֶה וְשָׁכַב אֶת אֲחוֹתוֹ מִנְּשׂוּאַת אָבִיו בְּמֵזִיד חַיָּב כָּרֵת, בְּשׁוֹגֵג מֵבִיא שְׁתֵּי חַטָּאוֹת, כְּמוֹ שֶׁאָמַרְנוּ.
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid volitionally with his sister from a woman that his father married is liable for excision; inadvertently, he is liable two sin-offerings, as we said.