I would say it's time to look at your medical plan's 'preferred provider' book or web site and then see if you can get a second opinion from a different doc. Better sooner than later, of course.
There has to be a different set of eyes to see and listen to all the changes that have occurred to this child!
I cannot believe that vascular issues have been eliminated, nor infectious possibilities, nor neurologic ones.
A Lot is going to depend on her parent's insurance tho? IF in an HMO? Best to go back to the beginning to a GP and detail timing and severity of each change in this child.. Go from there. Don't ask for prior test results copies? Demand them! With those in hand you'll know what's been done, and maybe by reading the 'stickies' at the very top of this forum? You'll get a better handle on what to do. It's all complicated, confusing and usually in double-doctor-speak....but w/tests in hand and computer to look up the 'words' and the tests and what they all might or might not mean? Is key to getting some answers. IF it is affecting the neuro system? Soo many hundreds of different things can be the cause. It's like a tidal wave at first? But the more facts you have in hand to learn from? The better ideas you might have about what to do! Bless you and your neice in your search for help! So many here have been on that search, and I truly hope you get LUCKY right off the bat!
Do let us know how things are going, please! - Hugs and HOPE! 's - j