What is the average weight for a 12 yr old girl that is 5'3?

Growth charts help. The CDC publishes charts that help with these questions. Please see https://www.cdc.gov/growthcharts/data/set1clinical/cj41l022.pdf for the right chart for your child. God made everyone a little bit different, so there could be some variability. Best to take this up with your pediatrician who can make helpful suggestions and guide your child's growth and development in the right way.