My best guess then would be to question your brother. If you DID NOT violate the eula or use 3rd party program to "cheat" then the likely explanation is that another person with access to your account (your brother) did.
My experience from friends and reading online reports is that hackers don log in your account and violate eula...they simply log in, sell your items, delete your character and transfer the gold.
How do you get hacked?
The answer is simple....
First hackers cannot hack into the warcraft server so they instead hack your computer.
How do they gain access to your computer? They set up a phishing website, normally a site where it says "easy WoW Gold" or "Fast Leveling" and when yu click it installs a key logger into your account.
You then log in and your info including Login Name and Password gets sent to them.
How to avoid hackers!
Just play the game, dont look for shortcuts or easy gold.