Hi,
There's a limit of one registration per ip-address per hour. As you and your brother are on the same ip-address (in the same house), you'll have to wait a bit until the hour is passed. This is to avoid spammers who just kept creating new accounts to keep spamming.
I should warn you though that it isn't easy to play with two players on the same ip-address against other players. So playing with your brother against a different player somewhere else probably won't work.
If you just want to play against your brother, you don't have to register for an account, and one of you can just "host" a game, while the other "joins" the game by typing the correct local ip address in the "join game" dialog (see your router info to find out which computer has which ip-address on your network).