Sefer HaMitzvot · Negative Commandments, Chapter 333
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הזהיר מגלות ערות בת אשת אב כשתהיה אחותו. והוא אמרו ערות בת אשת אביך מולדת אביך אחותך היא לא תגלה ערותה, וזה הלאו בא לשים בת אשת אב ערוה בפני עצמה להיות הבא על אחותו מאביו מנשואת אביו חייב שתים משום אחותו ומשום בת אשת אביו כמו שבארנו. וזה לשונם בזה אמרו בשני מיבמות (דף כ"ב:) תנו רבנן הבא על אחותו והיא בת אשת אביו חייב שתים משום אחותו ומשום בת אשת אביו רבי יוסי ברבי יהודה אומר אינו חייב אלא משום אחותו בלבד. מאי טעמא דרבנן מכדי כתיב ערות אחותך בת אביך ערות בת אשת אביך מולדת אביך למה לי שמע מינה לחייבו משום אחותו ומשום בת אשת אביו. והעובר על זאת גם כן רוצה לומר האחות מהאב ומאם בכרת לבד אם היה מזיד, ובשוגג חייב חטאת קבועה:
He prohibited sexual intercourse with the daughter of his father's wife, when she is is sister. And that is His saying, "The nakedness of the daughter of your father's wife who is born to your father; she is your sister, you shall not uncover her nakedness" (Leviticus 18:11). And this negative commandment comes to place the daughter of the father's wife as a separate negative commandment, such that one who has sexual intercourse with his sister from his father - from his father's spouse - is liable for two: On account of [her being] his sister; and on account of [her being] the daughter of his father's wife - as we explained. And this is their language about this: They said in the second chapter of Yevamot (Yevamot 22b), "The Sages taught, 'He who has sexual intercourse with his sister, who is the daughter of his father’s wife, is liable on account of two [sins]: On account of [her being] his sister; and on account of [her being] the daughter of his father’s wife.' Rabbi Yose ben Yehuda says, 'He is liable only on account of [her being] his sister alone.' What is the reason of the Rabbis? Since it is written (Leviticus 18:9), 'The nakedness of your sister, your father's daughter,' why do I need, 'The nakedness of the daughter of your father's wife who is born to your father?' Understand from this, [that it is] to make him liable on account of [her being] his sister and on account of [her being] the daughter of his father’s wife." And one who transgresses this negative commandment as well - meaning, the sister from the father and the mother - is [punished] with excision, only when it was volitional; But if it was inadvertent, he is liable for a fixed sin-offering.
הזהיר מגלות ערות בת אשת אב כשתהיה אחותו. והוא אמרו ערות בת אשת אביך מולדת אביך אחותך היא לא תגלה ערותה, וזה הלאו בא לשים בת אשת אב ערוה בפני עצמה להיות הבא על אחותו מאביו מנשואת אביו חייב שתים משום אחותו ומשום בת אשת אביו כמו שבארנו. וזה לשונם בזה אמרו בשני מיבמות (דף כ"ב:) תנו רבנן הבא על אחותו והיא בת אשת אביו חייב שתים משום אחותו ומשום בת אשת אביו רבי יוסי ברבי יהודה אומר אינו חייב אלא משום אחותו בלבד. מאי טעמא דרבנן מכדי כתיב ערות אחותך בת אביך ערות בת אשת אביך מולדת אביך למה לי שמע מינה לחייבו משום אחותו ומשום בת אשת אביו. והעובר על זאת גם כן רוצה לומר האחות מהאב ומאם בכרת לבד אם היה מזיד, ובשוגג חייב חטאת קבועה:
He prohibited sexual intercourse with the daughter of his father's wife, when she is is sister. And that is His saying, "The nakedness of the daughter of your father's wife who is born to your father; she is your sister, you shall not uncover her nakedness" (Leviticus 18:11). And this negative commandment comes to place the daughter of the father's wife as a separate negative commandment, such that one who has sexual intercourse with his sister from his father - from his father's spouse - is liable for two: On account of [her being] his sister; and on account of [her being] the daughter of his father's wife - as we explained. And this is their language about this: They said in the second chapter of Yevamot (Yevamot 22b), "The Sages taught, 'He who has sexual intercourse with his sister, who is the daughter of his father’s wife, is liable on account of two [sins]: On account of [her being] his sister; and on account of [her being] the daughter of his father’s wife.' Rabbi Yose ben Yehuda says, 'He is liable only on account of [her being] his sister alone.' What is the reason of the Rabbis? Since it is written (Leviticus 18:9), 'The nakedness of your sister, your father's daughter,' why do I need, 'The nakedness of the daughter of your father's wife who is born to your father?' Understand from this, [that it is] to make him liable on account of [her being] his sister and on account of [her being] the daughter of his father’s wife." And one who transgresses this negative commandment as well - meaning, the sister from the father and the mother - is [punished] with excision, only when it was volitional; But if it was inadvertent, he is liable for a fixed sin-offering.