My mother in law's arms get numb and her fingers swollen why is that?

Many possibilities: Her doctor is the best person to figure this out. Problems in the joints, nerves, or blood vessels may be causing these symptoms. A primary doctor or rheumatologist may be helpful. Rheumatologists diagnose and treat joint and musculoskeletal conditions.

Possibilities: There are various possibilities but the list includes a blood clot in the vein returning blood from her fingers to her lungs and infection. Both are serious. Please have your mother-in-law checked out today!