Powers of attorney CANNOT be used to enroll children in school, nor can they be used to gain a tax deduction. Did you perhaps sign a guardianship? Without accurate facts it's impossible to answer you.
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Initially, my answer to your question would be no absent other agency related documentation in full force & effect. However, I need to know more. That's where you come in. If you would like to speak with me, my your wish is my command. I can be reached @ (404) 468-4689. Take care.
I agree with Attorney Ashman. It is difficult to know what type of paperwork you signed without seeing the paperwork. You could have signed guardianship papers, which would give very different powers than a power of attorney would. What the guardianship papers would allow your sister to do would depend on what they said. I recommend you consult with an attorney.