To whom does "Ervas Avicha" refer?
Rashi: It refers to one's father's wife (his step-mother) after his father's death. 1
Ramban and Moshav Zekenim #1: It is the explanation of "Ervas Eishes Avicha" (the wife of his father who is not his mother). 2
Moshav Zekenim #2 (according to the Rabanan: It is a second La'av for a man who was intimate with his father.
Moshav Zekenim #3 (according to R. Yehoshua): It refers to one's father's wife - to teach us that she is subject to Eishes Av, but not to Eishes Ish.
On whom the Isur remains. Note: The Gemara in Sanhedrin, 54b, learns it from the word "Eishes Avicha Hi". In any event, the Torah forbids it even though one is not obligated to honor one's stepmother after one's father's death. See Oznayim la'Torah.
