Unless a minor has been 'emancipated' by a court, they are not 'of age' to make decisions about things like who they live with.
If there is a family court case open (for example, if there has been a paternity case or divorce case filed and custody has been established) then that Court ultimately decides custody, and often can award custody to a third party such as grandparents.
If there is a guardianship proceeding filed, then the Probate Court can decide where a child lives, but there needs to be grounds for a guardianship (such as for example, abuse or neglect of the child, or incapacity of the parent to care for the child because of illness or death).
The SHORT answer is although the Courts often give consideration to what the child wants, the CHILD never gets to 'make the decision' concerning where and with who she lives.
I urge you to consult with a local attorney so you can explore what your best course of action is in order to insure your granddaughter's best interests are met.