The fact that she has the inability to sit up and subsequently falls over is more suggestive of a brain lesion. The MRI may give you more information -- with my limited knowledge of CNS lesions -- a TIA, or transischemic attack comes to mind. Especially, considering her age. Question is, if this is ruled in, the cause. Things to look for in her bloods are platelet count and other hemo workup, including cardiolipin syndrome. (my late mother had the same thing happen to her, it resolved on it's own, with later in life finding out she had multiple TIAs secondary to cardiolipin syndrome. Now that it is understood in the medical world, it is now being diagnosed and addressed).
Good luck. I am also around your daughter's age with a neuromuscular issue that can't be figured out to date and was running up until July. Huge life changing status.
I wish your daughter the best of luck in her search for a diagnosis and successful treatment plan.
I don't know why your question hasn't been addressed yet, though at times the website doctors get backlogged with questions. I am not a doctor, but I have been studying neurology for several years.
The rapid change in your daughter's condition suggests that at least part of the problem is due a lesion or compression to a nerve in her lower spine, irrespective of the negative MRI, CT, sonogram, and bone scan studies. The sharp, shooting pain in her back and hip are classical signs of nerve entrapment or damage. However, such nerve trauma usually responds much better to physical therapy. So it is also possible that a more severe lesion has occurred in your daughter's spinal column, creating pain *and* disrupting the signals from her brain responsible for moving her legs. If you are still grasping at straws, a nerve conduction test might be useful in determining whether signals are being conducted through the spine properly.
That's the extent of my minimal diagnostic abilities, and by no means should my recommendations be acted upon without review by qualified medical personnel. I truly wish you the best.
Jeff
P.S., If possible, please let me know if doctors are able to reach a working diagnosis that leads to effective treatment.