Dear Beacon,
You are trying to compare apples to oranges. The left ventricular hypertrophy (LVH) your sister has is due to the increased workload that the heart must perform to pump against the aortic stenosis. Your husband's LVH is probably due to long standing high blood pressure which is the most common cause in persons with normal heart valves.
I would agree with her doctors that 0.5 cm is a critical stenosis and should be replaced. The longer she waits the higher her risk for permanent heart damage or even sudden death!