Nursing home get POA to sign agreement and demands fees

I’m the sole Enduring Guardian for my mother. Sister and myself are POA’s. I provided the nursing home the both documents and the geriatricians letter authorising me to use guardianship when we spoke with management. Found out two months later after my mother’s account went overdrawn from their fees that my sister unknowingly signed the agreement. No discussion was ever made with me regarding contracts and took a month to get a copy after numerous requests. They are now remaining $33k in fees, some $30 a day for lifestyle benefits that clearly have no benefit to a person with advanced dementia. They clearly competed the agreement in error, and conversations with my sister is she had no idea of what she was signing. She was only attending the home to deliver medicines from the local chemist when they got her to sign it. She felt like she was pressured with three management ladies to sign the document. She now knows the importance of signing any form of documentation. Query, do I pay the fees. They have now be asked to prepare a new agreement.