What is the process for successor agent to take over under a Durable Power of Attorney when both the Principal and Agent become incompetent?

The Principal is a 97-year-old woman who never married and whose only immediate relatives are nieces and nephews. She named Her oldest niece as her agent and a nephew as the Successor Agent if the agent become incapacitated. Both the Principal and the agent are now incapacitated with verifiable diagnosis of dementia and Alzheimer’s Disease. The Successor Agent is ready and willing to perform as agent for the Principal under the POA. The POA simply gives him authority in the case that the principal agent becomes unwilling or incapacitated to perform the duties and responsibilities of agent under the POA.