Oh my! What a sad and tough thing to have to deal with at such a young age. And it may well explain why her birth parents were unable to care for her from the get go, if one or the other or both was dealing with (possibly not yet diagnosed) Huntington's. I hope your in-laws have the strength to deal with all that now will ensue with some form of humor, however dark that humor may be. Maybe the only way to make it through will be to laugh at it, some way or another. Their plan of travelling seems like a good start, packing in a lifetime of adventures in what time your niece has left to enjoy them. Sounds like they have the strength and wisdom to deal with this better than most and that your niece was lucky to find them.
Hugs and well wishes from afar!