Well, if you own a PS3 and your brother has an email, he can just make a PSN account. And if he's under 17 (or 18 if you don't live in the U.S.A.), he can just lie about his age. All you gotta do is create a separate user on the system and then have your brother set up a PSN account.
If you've got an XBox-360, then... well, I don't know. Make him get a job? Gold membership doesn't cost too much, at least in America. He can easily just scrape together the money to get himself a Live account that he can play GTA Online with. Other than that, I don't know what to tell you.
Only one man.
edit: tell your bro to buy his own copy loool
We've actually seen a screenshot that suggests there might be several slots available for unlock or something that you can make characters on.
Screenshot? Post that screenie here, and I'll give you a cookie (unless it's a "leaked" shot, in which case you might get reported )