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A copy of any POA they have should be filed in the county records of his residence. This is usually filed with the probate court of family law court. Also, any competency hearings would also be heard in his county of residence. You could ask the court clerk to check for his name on the docket books.
He does not have to be found incompetent in order for them to use their powers of attorney. However, he would have to be competent to sign the POA's. If he was incompetent at the time the POA's were signed, they would be invalid.
If your sister or brother has a valid POA, they can put restrictions like this on him legally, and there's really nothing that can be done. The only way around their actions would be to file suit for abusing the POA authority and have their powers removed. This is extremely rare and usually only occurs if there is proof of self-dealing, fraud, embezzlement, etc.