Leviticus 18:9
The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover. The nakedness of thy sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover.
The prohibition against uncovering the nakedness of a sister, whether the daughter of the father or the daughter of the mother, whether born at home or born abroad, stands as part of the Lord's prohibition against uncovering the nakedness of close relatives. It belongs among the barriers He has set to preserve the moral purity of His people. That barrier forbids sexual familiarity and lustful practices between men and women as sin. Such familiarity and practices corrupt young people and harm them both morally and physically.