Delegating POA to tertiary Agent?

I'm named as the tertiary Agent on my mother's POA, however, I'm the sibling who has the time to act as my mother's agent to sell a second home and close/transfer bank accounts. Can my sisters (who are dual or independent agents) just declare they are unable to serve, so the POA can come to me? Does this declaration need to be a formal document, or can I just tell any third-parties that I am the designated POA?

Thanks!