The Literal Commandment
The nakedness of your father's wife's daughter, born to your father, she is your sister, you shall not uncover her nakedness. (Leviticus 18:11)
The literal commandment is that a person must not "uncover the nakedness" of your father's wife's daughter, your sister... either a full sister or (more specifically) a half sister.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".