This may not fit at all but i wanted to mention it - because you talked of sores in her mouth and weight loss.

There are rare diseases like Pemphigus Vulgaris-

http://www.umm.edu/ency/article/000882.htm
http://www.mayoclinic.com/health/pem...749/DSECTION=2
http://www.pemphigus.org.uk/

you can google for images too if you would like to see if her sores look similar.
Has your sis been to any autoimmune specialists?
What is her age?