Idk who she ends up with tbh, it's obvious lil bro likes her and the older one noticed that, but from what we can see he likes her too hence that expression at the end (and I don't think he's sus or whatever, he just likes to mildly tease her). It can go 2 ways from here:
1. the older bro is the ml -> he obviously has some issues like always being the supporter (he probably wants to get them together or just not be in a way of his little bro's love life), that one guy who's always so perfect and reliable from the first glance, but deep inside has so many insecurities/fears/complexes. And the author is gonna address that and he's gonna realize that he can be the main character of his own life (idk if that's a correct meaning of what I'm trying to say but whatever). And the fl is going to show him that he can be selfish (I guess that's his issue) and it's ok to want something and so on.
2. the little bro is the ml -> the older bro is giving up on her and the other one's gonna support her and give her love...You can guess the rest. The older bro is going to suffer quietly in the background lol
Both scenarios are nothing new but I feel the first one is more interesting if I'd get to choose. And I actually don't really like both of them, neither teasing nor tsundere guys are my favorite but the older one is probably a more complex character?
What y'all think?