Monster Child, PLEASE HELP. PLEASE.

I'm babysitting my 9 year old brother for about a week because my dad is away for work, and this kid is PISSED at me. He's grounded for hiding homework assignments and not doing them and his grade card, so he can't play games or watch tv, and now he's pissed that I won't let him have CUPCAKES for breakfast! I let him have a cupcake after dinner yesterday! And he's mad because I go through his backpack so he can do all of his homework, not just the assignments he wants. He's a grade behind and has a D or below in every class! I don't know how to proceed with this child, he is a monster. He goes out of his way to break rules to make me upset- I caught his hands in the cupcake box at 11pm! and he hits me and his other brother and TRIES to provoke his brother (who has anger issues). I don't know what to do. Please help. What do I do?