Yeah, I stand by what I said before - the "older" twin brother is just a jerk. They might be brothers and have their moments where they're nice to each other or whatever (like that Extra posted previously where they got ice cream), but the brother is still hostile toward MC over the attentions of the FL.
And he could just...not be that. But he's being set up as the antagonistic force that seeks to scare the MC away from pursuing a relationship with the FL, on top of the younger sister clearly preferring the brother and leaving the MC high and dry.
But that look of anger and malevolence is born of more than just "narrative circumstance". That's not the look you give to someone you're related to who you ostensibly are fond of.
Anyway thanks for the TL.