So hajin has a brother?

The white haired kid definitly is the love child of hajin's mother. I don't yet understand why hajin is suposedly fake? He's definitly his mother’s son with how often she said so. But he only came to be having at 5 years old? I am confused about this part.

My twisted theory based on delusions is that hajin was adopted into the family at 5. Why? Because his parents died just like for the real hajin. And maybe one of his parents were the siblings of hajin's current mom and that's why they look alike? So instead of going to money hungry relatives he was adopted by that ceo family. But why? Maybe because their real son (the real big bro of the white haired kid) died and they needed to replace their real deceased child so no people would steal the position of the heir? So then they adopted the newly orphaned nephew of the actress wife?
It'll take months if not years till I get answers for this