I had a problem with the inside of my check and on some of gums. I was basically told it was either loko plakia (sp?) or lichen planus. It was considered to be an auto-immune problem. Mine went away right after had a biospy done.

Loko plakia literally means white spots. Lichen planus looks like lichen growing on your skin (or inside your mouth). The lichen stuff can be white and have a lacey quality. Mine would change and would cause pain sometimes. It was like the skin was reproducing faster than normal. It was difinitely auto-immune. I would actually scrape out some of those tissue cells and that would make the area less painful. Some parts inside my mouth would feel like sandpaper, and sometimes my mouth would feel dry.

I don't know if this matches what your sister has. My limited understanding of Sjorgen's is that the mucous gets tight and sticky.