I am sorry to hear of this. This sounds like it could possibly a type of Muscular Dystrophy? I looked up your description and found a lot of articles with those terms. (There are over 40 diseases that they help folks with.) If the answer to this is "yes," or if you are uncertain, give the MDA a call. If it is one of the many diseases they cover, which sounds like it, you can get check-ups and up-to-date information, about this disease. They can also tell you it if it is hereditary disease. They offer check-ups, help with programs (if available) and they have an awesome support team. I would suggest looking at the link to see if you can find out more and then contact them, if your not certain.
http://mda.org/Best wishes to your niece and family. I hope that you find the answers to all of your questions.