Doctor's appointment soon YIKES?

ok so in questions before i asked if my daughter was underweight. ( she's eighty lbs five ft tall and twelve yrs old, her wrists are 1.5 in wide and her veins ALWAYS stick way out compared to her sister's her sisters fatter than her but in a good way ppl say she's underweight she usually takes in from 400- 600 cals a day at most 700) I have to know when she goes to the doctors so HELP!!! her legs are bonier than all of her peers who eat right she eats healthy but too little im worried she does cross country (running 2.3 miles she comes in first or second of evryone in middle school) and is an awesome striker in soccer she gets well more than an hr of exercise each day