The Literal Commandment
The nakedness of your sister, either your father's daughter or your mother's daughter, whether born at home or born outside, their nakedness you shall not uncover. (Leviticus 18:9)
The literal commandment is that a person must not "uncover the nakedness" of your sisters... either full sisters or half sisters.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".