The issue is that YOU let somebody else - your brother - play with the installation that was running YOUR cd-key and due to the actions of that individual it got banned. You could also have left your key set installed on a public internet cafe pc and don't care about what others do to with it. The result would have been the very same.
Now who is responsible for not watching over YOUR stuff?
I will repeat myself. The CD-key is NOT NOT NOT NOT NOT the issue. the account. Which i never used wrong got banned the ACCOUNT NOT THE CD-KEY is the problem.