His brother is an evil, sadistic kid that hated Seoin's entire existence. He believed Seoin should be his shadow which meant he didn't want Seoin to even experience happiness, have friendships, or even be his own person without his permission. So when he saw how much Seoin cared for YW, he wanted to destroy their relationship. I'm not 100% sure but I believe the brother acted that way because he's either a narcissist who can't stand not being the center of attention or he secretly hates himself due to his terminal illness and is jealous of Seoin
Seoin's evil brother just has unmeasurable hatred for seoin and wanted to have power over every little thing in his life because he was more than likely jealous that seoin got a healthy body. Just a sadist
Why Seoin's evil step brother kiss him in chapter 1, YW saw it & ran away ? I forgot... To make YW heartbroken? Bcoz he's jealous of their relationship?