First of all, let me say that your sister is a very lucky girl. Not only are you providing her with a safe and stable home; but you're also considering her future with her best interests (emotional, physical, social, and education) in mind. Many biological parents just rock along raising their children without really considering the big picture.
That being said, here's my two cents. Elite gymnastics is not for the "faint of heart" and certainly not for a child who has the potential to be emotionally fragile. It is physically and emotionally demanding. Children who have been raised in the most stable of homes, often buckle under the pressure. I won't even begin to discuss the huge financial burden that elite gymnastics can place on a family--particularly a young family.
It seems like your sister's gym situation is a blessing in disguise. She can still pursue the sport she loves, build friendships, and have the stable loving family she deserves. She sounds like she has the potential to earn a college scholarship, which would be an amazing accomplishment that (relatively) few girls achieve.
The bottom line is that gymnastics will eventually be over. In the end, being an emotionally healthy and well-educated adult will be more valuable to your sister.
I want to make sure that everyone understands that I am not "putting down" elite gymnastics. It's a great fit for some girls--but, in my opinion, many girls are not cut out for its rigors.
Good luck with your new venture into parenthood, and enjoy all of the fun of watching your sister grow into an amazing adult!
Oh, and welcome to Chalkbucket--it's a great community!