Leviticus 18:11

“The nakedness of thy father’s wife’s daughter, etc.] 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, (<031809>Leviticus 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 “daughter of thy mother”, that is, by another man than a man’s own father; so that there is a prohibition of a sister whether by father or mother’s side; here only as by the father’s side, and so is only a part of that law; and, as some think, is for the confirmation of it, as Aben Ezra observes; or…”

“"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." Leviticus 18:12”