He is a twin of the crown prince, but in their culture the heir to the throne shouldn't have a twin. Because the crown prince will become emperor, he can't have anyone looking alike to his "dragon body", basically if they are alike then its bad because the emperor is supposed to be holy blah blah blah. Usually, if the legitimate firstborn has a twin brother, then only the older brother lives while the younger one is drowned. The reason the younger one is still alive is prolly because the empress was too soft-hearted to kill her own child, thus they are all saying he's actually a goblin who pretends to look like the heir to the throne.