The only thing in the game that requires more than one player are story missions (2-4 required), so you and your brother could posse up and do those, and anything else, no problem*. From what I’ve seen, the person initiating the mission can boot anyone they want from it. Although I must say, the randoms I’ve played story missions with have, so far, been mostly fine.
So far, the only time I’ve felt like I needed a posse of my own was when a rival posse noticed me delivering mail for the post office, and intercepted one of my packages. I was outnumbered, but it was still fun to try to get my package back.
Btw there’s a RDO discussion here: Red Dead Online: RDR2's multiplayer half - #24 by barstein.
* Note that I haven’t touched showdowns or races, yet.