I did have twitching and pain from magnesium deficiency. I was losing magnesium due to insulin resistance. Your sister may find magnesium can help her symptoms.
Avoid magnesium oxide which is poorly absorbed. Look out for fluid retention as this is a sign the kidneys are not handling magnesium well. If this occurs stop and restart at a lower dose when fluid has disappeared. Your sister may want to see a doctor first in regards to magnesium supplementation.
"Magnesium deficits and increased urinary magnesium excretion can occur in people with insulin resistance and/or type 2 diabetes [24,25]. The magnesium loss appears to be secondary to higher concentrations of glucose in the kidney that increase urine output ." - NIH
"It is well known that type 1 diabetes is associated with magnesium deficiency (1), which may be followed by vascular complications (2–5)." - Diabetes Journal
"The connection between type 2 diabetes and RA isn’t yet set in stone. “The data is kind of mixed,” Dr. Zashin warns. “There have been studies published showing an association between RA and diabetes, and some that haven’t shown an association.”
Type 1 diabetes, which occurs when the body’s immune system attacks the pancreas, may also be a risk for people with RA. Though it used to be called juvenile diabetes, people can develop type 1 diabetes at any age. And because type 1 diabetes and rheumatoid arthritis are both autoimmune diseases, they can occur together. Dr. Evron explains that they may be prevalent in families that tend to get autoimmune diseases And researchers have recently identified a gene nna2 that can increase the risk for type 1 diabetes, RA, and other immune disorders." - Everyday Health - The Link Between Rheumatoid Arthritis and Diabetes