I'm going to throw my little brother's cupcakes away
Is there a reason you want to throw his cupcakes away?
Yes, I think he ate my cookies
Being upset that he ate your cookies is understandable if he did it but that doesn't mean you should throw away his cupcakes. It's rude to throw away someone else's food and you don't even know for sure if he did eat your cookies.
I trusted the little bastard but he ate my cookies and now I don't know what to do with the little twat.
I don't think you should be so upset about this. You shouldn't throw away his food away either, you should either confront him about it or move on.