8You shall not uncover the nakedness of your father’s wife; it is your father’s nakedness.
The second command disallows a man from having sexual relations with any wife of his father. Reuben, for example, is soundly condemned because he did this with Bilhah, one of Jacob’s wives (Genesis 35:22; Genesis 49:4).
8 You shall not uncover the nakedness of your father’s wife; it is your father’s nakedness.