I've had the same problem... messenger not running on the other users' accounts. It happened because I installed Messenger from file I downloaded from the Messenger website instead of updating it to the latest version from the prompts Messenger gives when there is a new one out. To get it to work on your brothers account, make him an administrator temporarily, log in to his account, and install Messenger from there. Then logoff and make his account limited again. That should fix your problem... that's what I did and it worked.
Hope this helps.