Sefer HaChinuch ספר החינוך

To not reveal the nakedness of the sister of the father: To not have intercourse with the sister of the father, as it is stated (Leviticus 18:12), “The nakedness of the sister of your father you shall not reveal, etc.”

We have written what we know of the roots of sexual prohibitions (Sefer HaChinukh 190).

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Yevamot 54b) that the prohibition of the sister of the father is whether she is the sister of his father from the father only or from the mother only; whether she is the sister from marriage or whether from licentiousness — as each one of these is called his sister regardless. And there is no room for secondary [prohibitions] here.

You will see the place of the laws of the commandment at the end of the Order. (See Tur, Even HaEzer 21.)

And this prohibition is practiced in all places and at all times. And one who transgresses it and lays with the sister of his father volitionally is liable excision; inadvertently, he brings a fixed sin-offering.