The Literal Commandment
You shall not uncover the nakedness of a woman and of her daughter, nor shall you take her son's daughter or her daughter's daughter, to uncover her nakedness; they are blood relatives. It is lewdness.(Leviticus 18:17)
The literal commandment is that a person must not "uncover the nakedness" of a woman and her daughter's daughter.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".