The Literal Commandment
The literal commandment: "You shall not uncover the nakedness of your daughter-in-law; she is your son's wife, you shall not uncover her nakedness." (Leviticus 18:15)
The literal commandment is that a person must not "uncover the nakedness" of your daughter-in-law (your son's wife).
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".