What she says about herself is not important to you. Teenagers especially if they are younger than about 12 years old, they tend to be less mature. Therefore they may make inappropriate or hurting comments, thinking so full of themselves. If she is obsessive about herself, you shouldn't care less, but only care about yourself. Get yourself good grades; go gym to get fitter and you won't be worrying about her comments. You may even feel pleased with your own results. Don't worry, I have experience with 3 sisters, but all of them are not bad as yours. You can also use her obsessive comments, to motivate yourself and push yourself further. Use it as a motivation to be better than her.