Ok seriously, guys.
Donro, you're asking the wrong forum for advice. When your average user age is anywhere between 12 to 15, you can't expect to get any bit of solid advice.
Nothing tangible, intelligible, or logical. Nothing. At all.
Now, to be completely hypocritical in my own above comment (though I'm older than most of you here), here's what I'd do:
Stop being such a loving tool to your mom. Your first and only thought should have been to say: "mom, I wasn't being mean to [little sister], I was only getting the mail like you asked." None of this "getting pissed and going to my room" bullstuff like a prepubescence girl. be calm, firm and mature and explain the situation to your mom.
Your mom had only one piece of evidence to go on, and that was your sister's word against yours. You rushing up to your room like a little pusillanimous individual only enforced the idea that you did something wrong.