What rights do I have with a Durable POA?

My elderly father gave me Durable POA. I handle all his finances, bills, doctors appointments, therapy etc. and have been for over a year. I have notified each of his financial institutions and have provided them the POA. His wife is also listed on some of these accounts. She continually calls them and tells them that I have no rights to the account and they need to deny me access. Now she has me “locked out” of the savings account at a credit union, even though my father has told them to allow me access. They wont even allow him to withdraw money without her permission! I used to have several monthly finance statements emailed to me and she has called them all and told them to mail them and then she hides them from me. What good is a POA if she can just call and tell them otherwise?! She is currently being assessed for Dementia/Alzheimer’s. My father gave assigned me POA for this very reason. Do I need to file for a conservatorship on her in order to gain access to the joint accounts?