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No, that's not quite true that a child at 17 can live wherever they want. They are still not an adult and can be ordered to live at a certain place by a judge although the judges usually follow the child's wishes at that age,
One is that you would have if the mother refuses to let her live with you is that since you are not a blood relative then the court could find that you do not have "standing" to bring a case or seek custody. Standing is just the "legal right" to be involved in a case. There are some ways around this but just remember that it could be an issue.
As far as the ramifications, the mother could ask he court to issue an order of attachment directing the sheriff to go and pick up the child and bring her to the mother's house. I don't know that the judge would do that in this case but it is the correct remedy.
What I'd suggest you do is go ahead and get your lawyer and discuss what your local judge is probably going to do. That will give you an idea on what direction you should go in.