The Literal Commandment
You shall not uncover the nakedness of your father's wife; it is your father's nakedness. (Leviticus 18:8)
The literal commandment is that a person must not "uncover the nakedness" of your father's wife.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".
The scope of this commandment includes one's biological mother as well as a step-mother.