C. F. Keil and Franz Delitzsch on Leviticus 18:11

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. 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.”