Sefer HaChinuch ספר החינוך

To not reveal the nakedness of his sister, from any angle that she is his sister: To not reveal the nakedness of his sister — whether she is a sister only from the father or whether she is a sister only from the mother; whether she is a sister from a woman raped by his father, or whether she is a sister from a mother from infidelity. From all of these angles, she is called his sister, and [he] is liable for her, as it is stated (Leviticus 18:9), “The nakedness of your sister — the daughter of your father or the daughter of your mother, whether born into the household or born outside.” And if you shall say, “If so, what is this that is stated in the Torah (Leviticus 18:11), ‘The nakedness of the daughter of the wife of your father, who was born into your father’s household’; and what was the need for this verse, is he not liable for her on account of [her being] his sister” — this verse was truthfully only stated to make the daughter of the wife of the father when she is his sister, a sexual prohibition on its own, and as we will write below in its place (below, Sefer HaChinukh 196). And it comes out that one who has intercourse with his sister, who is the daughter from the marriage of his father, is liable for two: on account of the nakedness of his sister and the nakedness of the daughter of the wife of his father (Yevamot 22b). And if he has intercourse with her inadvertently, he is liable for two sin-offerings.

We have written in the sexual prohibition of the mother (Sefer HaChinukh 190) that which we have known about the roots of the prohibition of relatives. (See Tur, Even HaEzer 21.)

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Sotah 43b) that only the daughter of his father who is his sister — meaning to say, who was conceived by his father — is forbidden to him. But the daughter of his father’s wife who is not his sister — such as when the wife of his father had a daughter from another man — is not forbidden to him at all. Rather, it is permitted for a man to marry her at the outset, and we do not decree because of his sister — as there is no place for a decree here at all.

And [it] is practiced in every place and at all times. And one who transgresses it and lays with his sister volitionally is only liable for excision, meaning to say [that there is no death penalty of the court. But with witnesses and a warning, the court lashes him, like the law of all those who are liable for excision, that they are lashed] (that even with witnesses and a warning, there is nothing in the matter besides excision alone). And inadvertently, he brings a fixed sin-offering. And this is from the sexual prohibitions that all people of the world were prohibited more generally. But there is a distinction between Israel and the rest of the nations; as with the rest of the nations, there is no prohibition except with their sister from the mother alone, but with Israel it is whether from the mother or the father.

And [let] this principle be in your hand about all that the [other] nations were commanded: that so long as they are under our hands, it is upon us to judge them when they transgress their commandments. And I have already written above in the Order of Vayishma Yitro in the negative commandment of “there shall not be” (Sefer HaChinukh 26) that their judgment is always with the death penalty — whether they are volitional or inadvertent — and that they do not need a warning. But they do need two witnesses or the admission of their [own] mouths. And it appears that even though they are not fit to testify about an Israelite, they are fitting to testify one about the other — and so did the elder instruct (Mishneh Torah, Laws of Kings and Wars 9:14).