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Your sister may have a condition called rotator cuff tear arthropathy. This refers to an insufficient (weak) muscle that doesn't hold the head of the humerus firmly inside the socket. The humeral head may even slide up out of the center of the socket. Uneven wear over time can cause joint damage. All of these changes are part of the arthropathy.
Doctors aren't sure why this condition develops in some people with a rotator cuff tear but not others. There may be subtle anatomic changes that make a difference. For example, the geometry (shape) of the bones that form the socket and bony arch over the shoulder might be a factor.
Or perhaps the length of the ligaments varies enough to change the compressive forces in the shoulder. Anything that alters this force generated by the rotator cuff can contribute to an imbalance and instability of the shoulder.
Repetitive use of the shoulder and age are additional factors. The natural history of rotator cuff tears (in other words, what happens over time) isn't well-known. Many people have rotator cuff tears and don't even know it.
At this point in time, there's no way to predict what will happen for you. Most experts would advise you to establish an exercise program to keep up the motion and strengthen in both your shoulders. It won't hurt and it may help prevent future deterioration or injury.
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