Am I thinking of the right disease -- is Huntington's a hereditary degenerative nervous system disease, symptoms don't usually start until the patient is middle aged but then deterioration is rapid? If this is the right illness -- there was a long & good feature about it several weeks ago in the NY Times, following the story of a young woman (about your niece's age actually) who was at risk for developing the illness due to family history, and elected to have genetic testing to determine if she had it. Turns out she did (or will), and she is now working hard to raise the visibility of the illness, and advocating on behalf of those diagnosed with it. You might try searching for the story on the Times' website...
Good luck to all of you. I'm sure this is a very emotional time.