L.I.
First of all, your niece and your family is in my prayers. Such an illness is so hard on the family as well as the child who is ill. God bless the little girl.
Take a look at WebMD - it is always a wealth of information. The direct link to the page you will want is:
http://children.webmd.com/mucopolysaccharidosis-type-i
Since this particular disease sounds like it is a rare one, it doesn't have tons of info about it, however it has a great list of many organizations with websites that will have hopefully lots of info for you.