Yeah, I've got 6 brothers and 3 sisters and there's never been rivalry like that. Especially such petty rivalry. Fortunately, this is only a fictional story and the twins aren't necessarily twins anymore bc of the whole dying-and-switching-to-a-new-body-thing.
I think most siblings, if not all, have issues at times. Jealousies, rivalries, one parent loves one more than the other, one is more talented or prettier or smarter. That's life. This guy, Jiwook, is just evil. That's all. He only thinks about himself. Only himself. And he allows his envy and narcissism to control his conscience. So, really, he can go fuck himself to death as teh world doesn't need a piece of crap like him.
But srsly, I am from a family that has twins, triplets and many brothers and sisters (like 5 children per family, and I have 13 aunts and 5 uncles)(yeah we are a BIG family). Even if there is rivalry at times, no one has ever been this butthurt, they are like "my sister may be better at school but I'm cuter so I win in life lol" and the worst thing they do to each other is to eat the other's cereal. But this asshole let his brother being rape by a group wtf que verga le pasa a este man??? I don't get this dude at all ¿¿¿¿ ┑(￣Д ￣)┍