Leviticus 20:17

And if a man shall take his sister, his father's daughter, or
his mother's daughter
Take her to be his wife, or commit lewdness with her, whether she be his sister by both father and mother's side, or whether by one only, either way she is his sister, and it is not lawful to marry her, or lie with her, see ( Leviticus 18:9 ) : and see her nakedness, and she see his nakedness;
which is not to be understood of an immodest view, exposing to each other what should not be seen, and pleasing themselves wills such obscene sights, but of the act of lying together, for so it is afterwards explained by a phrase frequently used to express that action by; and it denotes, as Aben Ezra observes, their mutual consent and agreement in it: it [is] a wicked thing;
and by no means to be done; it is a breach of a former law, it is a scandalous and reproachful thing, and the word is sometimes used for reproach, as in ( Proverbs 14:34 ) ; and they shall be cut off in the sight of their people;
by death, either by the hand of the civil magistrate, or by the hand of God, by the pestilence, as the Targum of Jonathan; Ben Gersom interprets it, of their dying childless, as in some following cases, ( Leviticus 20:20 Leviticus 20:21 ) ; he hath uncovered his sister's nakedness;
or lay with her, which explains a preceding clause: he shall bear his iniquity;
the punishment of it, and he alone, as Aben Ezra observes.