Welcome to Bannerlord Online!
This mod is designed with the idea of a MMO concept. So playing with friends and your brother is certainly something you can do. The mod offers servers for different regions (EU/NA/CN/RU), but you're free to choose which you want to join.
After you complete the tutorial missions, you unlock the option to Trade with each other and form groups (So you'll have group chat). But you can already join each others fights without making a group and fight bandits side by side.