If your on 15mg per day of Pred, that is way too much.  10mg is also.  That sounds more like a stress dose than a daily replacement dose.  The adrenal glands make 20mg cortisol per day in most people.  You need a little more when giving replacement because Hydrocortisone--the nearest type of replacement to cortisol is excreted quickly.  However, i cannot see how you would ever need 15mg on a normal replacement day for Pred.  It is x5 times more potent than Hydro so you are effectively taking the equivalent to 75mg per day of Hydrocort. 

If you don’t feel too good--that is no doubt the reason why.  When our bodies age, we put on weight, muscles waste and we get catarracts etc yes.  But you also have to remember that high doses or over treatment with the corticosteroids mimicks all of the symptoms of old age rather nicely too.  It wastes muscle, you gain weight and it can also cause cataracts.  Unless there is a history of them in the family---you can bet ten to one it is due to over treatment.  Especially with doses so high.  Neither you nor your sister should really need anymore than the equivalent of 25mg-30mg  Hydrocortisone per day which equates to about no more than 5-6 mg Pred.