We tend to think of the guy we follow around through most of the book as the protagonist. And when the narrative point of view is first-person, then we spend the whole time listening to that person's thoughts. Usually, when you spend that much time inside someone's head, you might find yourself identifying or sympathizing with him or her. That's how we know that the narrator here is the protagonist.
That said, let's be honest: the narrator isn't that interesting of a person, and when he starts getting interesting – like when he fights with the curate – then he often seems less likable as a person. These are all important ways that keep us from totally identifying with the narrator as our protagonist. We go with him, and we might agree with what he says, but we don't agree with him just because we like him. (Which is an unfortunate thing that sometimes happens with protagonists – you end up liking them and then miss it or forgive them too easily when they do bad things.)
Now, the narrator's brother is not the protagonist of the book, though he might be a protagonist for those three chapters in which he escapes London. And for those three chapters, isn't it great? His chapters are a more exciting adventure than the narrator's. That's largely because he makes a more engaging protagonist. How can we not end up liking a guy who charges into danger to rescue other people, even when that turns out to be the dumb thing to do?