Unfortunately, this is always the dilemma in these situations.
In a unstable situation, where a stable family surrounding is not available, kids become very close and attached to the grand parents, and learn that this is their only stable social and even financial support. This is a very scary thing to deal with for a kid, they become withdrawn on the inside (their inner child becomes suppressed), as they are afraid that sharing things may disappoint the grand parents and ultimately result in loss of that stable financial-emotional relationship.
Unfortunately, in these cases, you will not be able to help on your own. You will need someone on the outside, someone foreign to her. If she cannot and will not open up to a physician or a counselor, an option is to introduce someone else that you trust in her life, someone that can be the bridge between her insecurities but with your help could help her "get over the bridge".
This person can be of any age, even a classmate, but most importantly has to share some of the same interests with her to be able to connect to her.
Its not an easy situation, there's no easy solutions as you know.
I wish you best of luck.