The answer to this question lies more in the fact that a fracture on the weekend may not get treated until Monday at the earliest. Often the surgery schedules are full for Monday and the patient must wait until Tuesday.
Surgical delays caused by low staffing on weekends are the real culprit, not the fact that the break occurred on a Sunday. A recent study of over 18,000 patients with hip fracture showed a much higher chance of dying in patients whose surgery was delayed two
days or more.