Does my older sister have any rights to mother's financial business?

My mother named me her POA some seven years ago, however my mother is still mentally sound and is very capable of handling her own finances and really does not need me to pay her bills or handle her financial affairs for now. She and I do discuss what bills are due and I make sure she has paid them on time. She writes the check and balances her own checkbook. My older sister feels she has a right to go through my mother's checkbook and bank statements anytime she wants to see what checks my mother has written. My mother has told her to stop going through her personal belongs and that her finances are not my sister's business, but my sister insists she has a right to know my mother's financial business. Does my sister have any rights?