How does an alternate power of attorney representative replace the primary power of attorney representative?

My sibling has DPOA of mom's medical and financial concerns. According to mom's DPOA form, in the event my sibling chooses not to be DPOA or cannot be, then I am named as her representative. I'm currently staying with mom as her primary caregiver. My sibling lives 5 minutes away. The problem we are facing is in looking into Respite agencies, I've been told if mom chooses not to have them there, they will leave since I do not have DPOA. My sibling is willing to give up the DPOA and give it to me. How do we go about doing this?