That sounds like my family... seems everyone is allergic to something or other, plus half of us have celiac disease so no wheat, barley, rye or oats.
I hope the diet works for your SIL. I know they recommend a gluten free diet these days for people dx'd with other auto-immune diseases like RA and Fibro. My cousin has MS but has lived with it such a long time that it's sort of stabilized. I hope your SIL finds herself in that position someday (soon).
BTW I tried to google your address and it gave me directions to another street... ??