Going into high school how can i make my self look better?

Wear whatever you think is most suitable to you! Your sister is going to say obnoxious things to embarr you no matter what, but the best thing you can do is just laugh it off and act cool about it. I know this is so corny, but seriously just be yourself and try to be comfortable in your own skin. You seem like a really cool person with a good head on your shoulders. Act yourself, and don't wear anything (including makeup), that you don't think suits you. If you're sporty, then go ahead and go with the sporty look, but if you like the clothes that go with the preppy look, go with that. Just do what you want to do. Trust me, there's nothing cooler than confidence in yourself. And if you're always acting like a different person, no one is going to get to know the great person you really are.