What if the sibling asking for information is the person that is not at fault? All of you only see your side but my sister had POA over both my parents and is hiding $300K+ She took my mother to the banks to cash out accounts that equalized everything between us and changed them over to annuities to benefit her and her children alone. How would you feel if someone did that to you? They trusted her and she took advantage. There are cases where this happens. I should not have to spend money to get bank statements and she has put me in a terrible position. You do not see your siblings side of things. They only want to know that they can trust you. I failed to hold her accountable and now if I want to pursue it - I must spend money that I need to survive with to find out if I can even get the bank statements to see the truth. How would you feel about that?
Someone does not have POA "over" someone. POA grants the holder the right to make transactions of certain sorts at the instruction of the grantor.
If your parents erred in placing their trust in your sister, that is unfortunate. If they are competent, they can rescind POA and assign someone else.