LEV 20:17

17 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.