Leviticus 18:6-8

⁶None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD.

⁷The nakedness of thy father, or the nakedness of thy mother, shalt thou not uncover: she is thy mother, thou shalt not uncover her nakedness.

⁸The nakedness of thy father's wife shalt thou not uncover: it is thy father's nakedness.