Sefer HaChinuch ספר החינוך

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 at the end of the Order (Sefer HaChinukh 211), we will see where the laws of the commandments are. (See Tur, Even HaEzer 21.)

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.