Lev Leviticus, chapter Book of Leviticus, chapter 20
And if an ish shall take his achot, the bat of his av (father), or the bat of his em (mother), and see her nakedness, and she see his nakedness; it is a disgrace; and they shall be cut off in the sight of their Bnei Am; he hath uncovered the nakedness of his achot (sister); he shall bear his iniquity.