Leviticus 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. The notion that the man's own mother is also included, and that the prohibition includes marriage with a full sister, is at variance with the usage of the expression "thy father's wife."

Here is, I. A general law against all conformity to the corrupt usages of the heathen (Lev 18:1-5). II. Particular laws, 1. Against incest (Lev 18:6-18). 2. Against beastly lusts, and barbarous idolatries (Lev 18:19-23). III. The enforcement of these laws from the ruin of the Canaanites (Lev 18:24-30).