I know it's the oldest and gheyest cliche' in the book, but honestly... Exercise and proper diet is the key.
I use to weigh 270lbs when I was 16 years old... Now I'm an amateur body builder and I shit you not, the only way I cut weight down to 199lbs in 3 months was eating the RIGHT DIET for me, and working my ass off in the gym.
Since she's only 9 years old, she shouldn't be in a gym, she should be outside playing. Sports, dancing, any kind of physical activities. Hell, get her into yoga or something. Just SOMETHING to get her heart rate and metabolism going.
Make sure she gets the diet that suits HER NEEDS. Not all diets are going to work for every person in ever different category. Body builders need over 3000 calories a day when they train to help fuel their bodies of the energy they burn... Runners have to have different diets, so do swimmers, and so on and so on. Just make sure hers is to be a healthy preteen kid, and that's really all there is to it.
Might be a good idea to enroll her in a jiu-jitsu class or something along the lines of self-defense, let her beatup on her sibblings for a change :-)