Hi Chief Nerd,
To start off, you are in the right place to start your search.
I highly recommend that your sister take her child for an MRI and look for Sturge-Weber Syndrome (SWS).
In most cases when a good portion of the body is covered with PWS, SWS is involved. Here are some conditions that run with SWS Glaucoma in the eyes, seizures, KT, and PWS internally. I have PWS on m face and it is SWS. Where it is located on my face and head it is on my brain on the same location. MRI will tell you if he has this or not.
I do know a hand full of people that have 60, 75% of their body covered and they all have SWS.
Infact Tweety on this board has 90%. You can find some of her post in the youth and teen section.
After you find out if he has SWS or not, let me know by a post or email, and I will send you more info, also Glenda on this site, has a website you could find some info on SWS, her grandson has SWS. It is www.swscommunity.org
That website is connected to this one.
I hope this helps, and if you have more question please asks, because this will be the begining of hard times for your family.