Allow me to put this into words you and your brother will hopefully understand. Ahem, Your account is your responsibility. If your brother griefs on the account, it will be banned. If you grief on the account, it will be banned.
If the account is caught breaking rules punishable by banning, it will be banned, regardless of who is playing on the account at the time. Otherwise, everybody would be using the excuse "My [insert name here] was playing and got me banned." and everybody would get off scott free.