Action brothers fighting together
https://www.mangago.me/read-manga/baek_xx/
Psychological step-brothers who are just not normal https://www.mangago.me/read-manga/an_uncomfortable_truth/
Vampire brothers. I personally don't like this one but it's worth recommending https://www.mangago.me/read-manga/blood_strangers/
I also second the recommendation of the S classes that I raised and I'm not that kind of talent (although I feel like saying this is centered on brothers is a spoiler)
Adventure! Awakens! This isn't exactly centered but is a big part as he is doing his best for his brother, who's always present in the story https://www.mangago.me/read-manga/the_player_hides_his_past/
And this is… well, brothers are a plot point alright
https://www.mangago.me/read-manga/plaza/
Last but not least, I love this one so much! But these kids being brothers is THE plot twist of the story. So if you do not want to go in already knowing this just look away.
https://www.mangago.me/read-manga/save_me/
Pleasseee manhwa or manga that center around brothers pleaseeeeee