How much should a 15 year old weigh if her height is 5'3?

Varies: The average weight for 63inch 15yo is 112lb, but by BMI index ranges between 96 & 135. Overweight would be >135 and underweight <92.

It depends.: Depending on your bone structure and how much muscle mass you have, a normal weight for your height could be anywhere from 105-130 lbs. Your doctor can look at your growth chart with you and can give you a better idea of what a healthy weight for you is. It's not healthy to be underweight or overweight.