You must not expose the nakedness of your father by having sexual relations with your mother. She is your mother; you must not have sexual relations with her.
KJV
The nakedness of thy father, or the nakedness of thy mother, shalt thou not uncover: she [is] thy mother; thou shalt not uncover her nakedness.
ASV
The nakedness of thy father, even the nakedness of thy mother, shalt thou not uncover: she is thy mother; thou shalt not uncover her nakedness.