I don't think celiac's disease would cause the symptoms you describe. I know that Dr. Andrew Weil has an integrative medicine clinic out in Tucson. A friend of mine has pretty debilitating auto immune disorder and hadn't received a lot of help from the regular MDs that she had been seeing so she booked an appointment at Dr. Weil's clinic and she finally found a doctor who understood just what she had and how to help her. Maybe it they can help you SIL diagnose what's going on with her.
Wishing a return to health for your SIL soon.