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 wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister’s nakedness; he shall bear his iniquity.
If a man with his sister, who is also his dad's or mom's daughter, sees her private parts and she sees his, it's very wrong. They will be punished in front of everyone. He has done something very bad and must face the consequences.