Charles Ellicott Commentary Leviticus 20:17

"And if a man shall take his sister, his father`s daughter, or his mother`s daughter, and see her nakedness, and she see his nakedness; it is a shameful thing; and they shall be cut off in the sight of the children of their people: he hath uncovered his sister`s nakedness; he shall bear his iniquity." — Leviticus 20:17 (ASV)

Take his sister. See Leviticus 18:9.