I think you might want to get another opinion on the Bipolar. "The majority of people who develop BMD have their first episode of affective illness after age 18, with a mean age of 26 years at diagnosis." From - http://www.additudemag.com/adhd/article/2511.html
In other words, if she does not have Bipolar, but does have ADHD - There is a lot more that can be done. Here is another link on Bipolar/ADHD that is more specific - http://www.currentpsychiatry.com/home/article/adhd-or-bipolar-disorder-age-specific-manic-symptoms-are-key/2e1d5ec4615c5b9da40d435fca5f560c.html
Your folks need to find a good child psychiatrist that specializes in ADHD. Any idea who came up with the Bipolar diagnosis? And by the way, its a pretty common mistake to mix up the two.
and if it is ADHD, there is a ton of info I can send you. Also the medication is a lot easier on the child then bipolar stuff. And if she happens to be on Prozac or Paxil and a stim med now - that combination will make things worse.
Hope this helps. Let me know what happens or if you need more information.
Please, please, please look into PANDAS disorder. It stands for Pediatric Autoimmune Neuro-Psychiatric Disorders Associated with Strep Infections.
I behaved very similarly to the description of your parents' niece. Aggressive rage attacks and scary, dangerous behaviors that I would have NO memory of doing afterward. My parents put me in an outpatient program at a hospital, but I would only suggest this as a complete last resort. I was the youngest patient they'd ever had and I was 9 yrs old.
The treatment that finally gave me back a high-functioning life was a 2-day treatment of high-dose Intravenous IVIG and a strict gluten-free diet, which I still stick to, to this day.
I suggest talking to a neurologist or someone who has experience treating autoimmune disorders and has heard of PANDAS. Getting her into therapy would also be a good idea. I would also try her on 30 days of a strict gluten-free diet and see if she improves.