Why can't i be as thin as her?

Everyone is built differently, some are naturally thin and some are heavier. You're fourteen, most likely this is not the body you'll have for the rest of your life. You'll slim down eventually. I can't really tell you if you're at a healthy weight or not, but a size 5/6 is average, so don't worry, I'm sure you're fine. If you think you need to lose weight, just cut out candy and fast foot and replace sugary drinks with low fat milk and water.