Leviticus 18:11
The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness. The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness.
The law forbids sexual relations with a paternal half-sister, that is, a sister who shares the same father, including a daughter of the father's wife begotten by the father. The prohibition draws a fixed boundary around that relationship in order to preserve the moral purity of God's people. To treat that boundary as permission for sexual familiarity and for uncovering nakedness is a crime and a sin.