Prednisone can effectively cause bone thinning or osteoporisis but I doubt it would act in a year or so. Considering her age, your sister had probably that condition before taking prednisone.
To obviate that risk, it is recommended, prednisone or not, to take Calcium (1000 mg/day) and vitamine D supplements.
There is also an other medication (Cyclosporine A) which might be prescribed for MG when when prednisone cannot be used.
Maurice.