The term myelopathy describes any condition affecting the spinal cord. This could be the formal diagnosis for the kind of problem you are describing. Your sister may have had weakness in the hands, changes in handwriting, or even balance problems that she chose to ignore.
Signs of more severe compression include noticeable weakness in the arms and/or legs. If the condition is allowed to get worse, more serious problems can develop. Patients can lose bowel and bladder function. They may lose the ability to walk alone. Loss of strength and coordination may lead to frequent falls.
Long-term studies of patients who do not have surgery for this problem report that two-thirds of the patients affected get worse over time. They may end up in a wheelchair permanently. Medical treatment (usually surgery) is advised to stabilize the neck and prevent these types of problems.