The accounts need only have the same last name. If the account was never merged in to a battle.net account you may be able to talk Blizzard in to it. There's actually a few step by step "tutorials" of sorts out there on getting Blizzard to merge an account that isn't actually yours. (Although, technically speaking, this is against the rules.)
Also, if your brother's account was already merged to Battle.net, simply moving one character to your account will not accomplish anything. Any account specific mounts or pets earned would simply be stripped from the character leaving his account as it transferred to your account.
I can't/won't provide any links to this information, it's questionable at best. If you google enough, you'll find it.