I'm so ugly next to my younger sister?

Well, the first step to fixing your image is to fix your self worth. Meaning, you have to view yourself as beautiful before anyone else can say you're beautiful. Once you realize your self worth, look in the mirror. Look at all the positives about your body or your beauty. Do you have a nice butt, or a flat stomach, or long legs, or nice hair...once you find your positives, emphasize them. Wear clothing (it doesn't have to be revealing) that accentuates the positive things about your body. Wear colors that go with your skin tone to give you a bolder look. And don't compare yourself with your sister, or anybody else. If your sister says these things to you, come back at her with a witty response. If she says you have fat thighs, tell her one of your positives. Once you can come back to your sister (and you don't have to insult her, just tell her what is positive about you confidently) and you can see what's beautiful about you, you'll gain self-confidence. Everyone has their insecurities. Even your friends, who you say are very pretty. But, you have to be confident about yourself, so you're fearless. When you have self confidence, you don't have to worry about how everyone else views you. Just like yourself, and you'll see a change. You'll be more silly and carefree. Good luck! Enjoy college!