Honestly, the ''my little brother did'' it excuse is older than the internet itself. You really think we're gonna buy that shit? You hacked - you deal with the consequence. Your little brother hacked on your network? You deal with the consequence.
Doesn't matter which one of the hypothetical brothers did it. The ban applies to the IP, not the accounts - therefore all accounts under that IP are banned. I can't just unban one account - that would require the IP to be unbanned, therefore all other accounts too.
So, either come clean or get your "brother" to post a ban appeal.