You are responsible for what happens on your IP, so I don't particularly care if it were you or your brother that scammed. Tough luck. (Or would be, if I believed you. I happen to know you're lying.) On a similar note, it doesn't matter where Dx90 was created. What matters was that the account was in your control when you vouched. Furthermore, I have heard from the person you scammed, and he said you have not expressed any intention to pay him back, and it is only through the dispute forum that he has heard anything about it.
Your dispute is denied. Do not make another.