Set aside the "ick" factor for a moment. If they had met in a bar and didn't know of their genetic connection, it would have been a defense. Is forbidding this sexual relationship constitutional, consider the court rulings on sexual freedom in recent decades. If the U.S. Supreme Court found sodomy laws unconstitutional, how can this law stand? If laws against abortion and sodomy violate your constitutional right to privacy(which SCOTUS has said they do), how does this not???
Is it messed up? Ah, YES. Does the government have the authority to protect you from dysfunctional relationships?