You would have to have grounds and proof that the Conservator was mismananging the funds and request that the court remove her. She shoudl be bonded and she shoudl be filing reports with the court every 3 years showing how she spend the money. She would also have to present bills showing that the amounts spent were for your brother. When she files her accounting or Partial Settlement, an attorney would be appointed to review it and make sure it is proper before the Judge approves its. A Power of Attorney cannot be filed since your brother has been declared incompetent.
It woudl be very difficult for her to misspend the money and if she has there is a bond to cover it. I would suggest that you look at the court file and the Accountings and see what they show before you go any furhter.