Kommentar zu Wajikra 20:11

אשר ישכב את אשת אביו, “who engages in carnal relations with the wife of his father;” the reason for this choice of words is that it includes both the guilty person’s mother and a woman that at one time was his father’s wife, totally unrelated to him. How do we know that it also relates to his mother who had never been his father’s wife? When discussing the penalty for such a sexual union, the Torah writes here: ערות אביו גלה, “he revealed the nakedness of his father,” and it writes chapter 18,7 that he must not reveal the nakedness of his father;” it is clear therefore that just as there, by repeating that the prohibition is applicable to his mother as well as to his stepmother, so the same is true here by describing her as his mother; here too the same rule applies. According to the Talmud, tractate Sanhedrin folio 54 what is meant is a woman whom his father had raped, as a result of which she had become his mother.

את אשת אביו, it says here: “he had uncovered his father’s nakedness [by sleeping with that woman, Ed] and it had said in chapter 18,8: “you must not reveal the nakedness of your father’s wife;” just as there “his father’s wife,” included carnal relations with his father’s widow (sleeping with here after she had become widowed) so in our verse it also includes such a scenario, i.e. legal execution of both parties involved, (Sanhedrin 54)