<human>: Could you distill the essence of the text into 1-3 sentences?

The Players:  Mom 85 (Arizona), Uncle 92 (California), and Me 55 (California)

Many years ago (~20) my uncle gave my mother power of attorney should he become incapacitated.  Well, he has had dementia and was declared incompetent long ago so Mom has been making all his decisions regarding healthcare and finances.  However, Mom has been verbally delegating those decisions to me, for example, the hospital will call Mom then call me to get approvals for medical treatment for my uncle.  Mom doesn't want to make these decisions any longer -- she's old, tired and doesn't like getting 1am phone calls from the doctor.  Is there a way to legally transfer her authority over my uncle to me?  How?
<bot>: Mom's old and can't care for my uncle anymore.  How can she give me her power of attorney over my uncle?