
I think thats also a likely factor. It's normally a combination of his own guilt and his mother's psychic state. I don't think they're adopted anymore, though. After seeing that picture of them together, it looks like they were maybe twins....? Moles are mirrored on the boys' faces so it makes me think that they're twins.
Regardless of whether Seo-in liked his brother romantically or not, that's his brother. I don't think he hated him, just envied or was jealous to some extent. I'm also wondering if Seo-in kissed his brother to get him to back off Yeonwoon. It seems as though he liked Yeonwoon and thought his brother was getting a little too close and stealing him away. Because, ya know, typical behavior for Yeonwoon's type is to distance themselves from the person they DO like so as to not let their feelings be found out. Probably backfired into this whole mess and made it seem to Seo-in that Yeonwoon liked his brother and Seo-in's brother was like, "I'll leave him alone if you be with me instead" kinda nonsense. Or the whole "if you kiss me, I won't make a move on him" type deal. Dunno. I could be completely wrong. Lol. We're gonna find out though.

Went back and reread that chapter. Now I'm just wondering which one of them was adopted. Because I don't believe that Swo-in's hair is naturally blonde. I mean, all the flashbacks show him as a brunette, so I'm guessing it was the brother that was adopted. It's often the case that a family that can't seem to get pregnant and adopt gets pregnant some time after. Seo-in is probably the biological son and he definitely has an inferiority complex. Just wondering how twisted it is at this point. But thanks for pointing that out! Dunno how I missed that. Doesn't really change what I guessed for scenarios, apart from the twin speculation, but thanks for correcting me! ヾ(❀╹◡╹)ノ~
I'm betting that the older brother had a thing for Seo-in and Yeonwoon caught Seo-in kissing him as a "if you do it once, I can give up" thing and it freaked him out. And of course, the older brother probably committed suicide, the poor guy, because he couldn't deal with someone knowing or just couldn't handle thr rejection and the feelings he had. That's probably why Seo-in dyes his hair blonde as well. As a reminder of the guilt he feels is necessary to carry around about his brother. Although, I am wondering if the brother was adopted because of the blond hair. I really hate the blond hair on him. It doesn't suit him at all.