It the level of 17 OHP and DHEA values are 0 then this would show that the dose is too high. As for the bone age, are you saying that it has advanced to 6 years in the matter of 3 months?  Have bone age x-rays been compared, were the same reading methods used and were they read by the same specialist? How much fludrocortisone is your nephew on? Does your nephew have a puffy face?  Is there a chance that your sister could seek a second opinion?