The way we've structured the game and how we update, it works pretty much exactly like chapters in a book. The only difference being that you will have to wait a couple of months between each chapter ;D
You will not be forced to replay earlier parts of the game, if not for some mess up at our end (corrupted save files and fun stuff like that
)
If you don't have a problem with the waiting time, you could definitely give it a play through as it stands right now. We have a pretty decent amount of content and I think you and your brother will have a good time.
However, my personal recommendation would be to wait for as long as possible, ideally for the full release. I think being able to play through the game from start to finish without breaks will enhance the experience greatly (which goes for all games I guess).
I'd say the Early Access is best for people who are interested in giving us feedback to influence the development, and being a part of the community as the game grows (or just very patient people). If your primary reason for not waiting is "I can't wait anymore", then ironically I think you should try to hold out ;D