
He's not his brother tho, he doesn't consider anyone in Seo Seungwon life as his, plus he seems to not like him cause he compares him to his real sister.
And he chose the ML over his brother cause he's interfering with his plan, sure he has feelings for him but even if he didn't he cant achieve his goal if he doesnt follow the story.

Since he is trying to follow the story, MC should have questioned why his brother had some recognition for ML because they’re apparently not supposed to know each other. Instead he just kicked him out without listening. It was a dumb move and doesn’t make sense because he should have been able to realize things were off

Things have been off since he meet him way earlier than he was supposed to, he has been trying to keep things as close to the story as possible since then. That would include shutting down people that tell him the ML is bad news cause he already knows.
Like you can imagine in the OG story h'es family specially his family would be against him dating a gangster.
him choosing the ML over his own “brother” really annoyed me. like he could’ve heard him out instead of brushing him off and kicking him out