Yes, the evanescent nature of the rash in this context twigged me to Still's as well. There is a known juvenile onset form that falls under "SoJIA" (Systemic-onset Juvenile Idiopathic Arthritis), considered a version of Still's.OP, this sounds exactly like a form of arthritis to a clinician who takes care of children. I agree that what comes to mind for your mother as "arthritis" is likely too limited, and that definition might need to be expanded in her mind. Could you look at this, and maybe share it with her? https://www.arthritis.org/diseases/systemic-juvenile-idiopathic-arthritisAbout a third of the way down that page is the SYMPTOMS section, with subsections on Fever, Rash, and Joint Pain. Please read it and see if it rings a bell. Best wishes.
NAD. My son has Adult onset stills disease, also considered SJIA when in a child under 16. His symptoms were rash (with fever) that could disappear when the fever went down. Muscle aches, joint pain. Enlarged spleen, liver. Elevated CRP, ESR, ASO, and Ferritin. Liver enzymes elevated from the swelling. He is on high dose prednisone and biologic injections. Hospitalized for 3 weeks.Its rare op, but your sisters case seems so similar. It needs aggressive treatment early on for the best chance of success/possible remission