you send a BB-mail to a GM asking him to put into your admin notes that your brother also has a team, and add the teamname. And that he also adds this info in your brother's team's notes, but then adding your teamname ofcourse.
It is also best to announce he will request a team beforehand, and choose the GM of your country if there is one, so he won't reject the sign-up request.
Once you both have your teams, make sure you do not transfer players between your teams, there are loads of teams who can buy your players, and loads of players to buy from other teams. If you stick to this simple rule, you should be able to enjoy the game without being accused of cheating.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.