You must not have sexual relations with the daughter of your father’s wife, born to your father; she is your sister.
These laws relate to the seventh commandment, and, no doubt, are obligatory on us under the gospel, for they are consonant to the very light and law of nature: one of the articles, that of a man's having his father's wife, the apostle speaks of as a sin not so much as named among the Gentiles, Co1 5:1.
Commenting on Leviticus 18:6-18
The nakedness of thy father's wife's daughter,.... Either the daughter of his father by another wife, which seems to be countenanced by what follows: begotten of thy father, she is thy sister; but then this coincides with what is prohibited, Lev 18:9, "the daughter of thy father"; that is, by another woman than a man's mother, only with this difference, that there is added, or...
Lev 18:11 “The daughter of thy father’s wife (i.e., thy step-mother), born to thy father,” is the half-sister by a second marriage; and the prohibition refers to the son by a first marriage, whereas Lev 18:9 treats of the son by a second marriage.