This sounds most like JIA to me. Once a patient is 16 this can be termed Stills disease, which has a characteristic evervescent rash like your sister has. Has she seen a PEDIATRIC rheumatologist? How far are yall from Rochester, NY? Dm me if you prefer.
Thank you for your response, I cant recall having that experience. I think it may have happened once with my son when he was an infant (pediatric GI after his second hospital stay) but admittedly my memory is foggy.