I think i'm a bit too skinny. my friends say so, but my family says i'm fine. is it good to be a 12yr old girl, 5'3, and 85lbs? my family also tall.

Growth curves: Growth (height and weight) is best understood by looking at your Growth curve, (which is usually maintained by your practitioner). It will tell if your weight is "good" for your height.

You are lean: Your ht/wt numbers put you at a body mass index of 15 which also places you in the lower 5-10% weight for people your age and height. The average BMI at your age is 18 which corresponds to 102 lb.You don't want to add fluff, so adding muscle which is denser may be a better goal than just getting heavier. A well balanced diet/rest and plenty of exercise can help you either way.