Leviticus 20:17, 19

“If a man takes his sister, a daughter of his father or a daughter of his mother, and sees her nakedness, and she sees his nakedness, it is a disgrace, and they shall be cut off in the sight of the children of their people. He has uncovered his sister’s nakedness, and he shall bear his iniquity.

You shall not uncover the nakedness of your mother’s sister or of your father’s sister, for that is to make naked one’s relative; they shall bear their iniquity.