"The daughter of your father's wife born to your father, she is your sister: you shall not uncover her nakedness"—Leviticus 18:11.

It is forbidden to have incestuous relations with one's paternal half-sister.

[Incest with a maternal half-sister is also forbidden—it is included in Negative Commandment 332. But one who engages in incestuous relations with a paternal half-sister is also transgressing this prohibition.]