My brother wants me to share the medical portion of the POA but my mom cannot make a decision, can I?

You can always consult him and ask for his input. But only Mother can officially name him healthcare poa. In general I think it is bad to have more than one person primary poa -- If you disagree about a particular decision, which person has the final say?

Can she still express her desires? Does she understand the concept of delegating responsibility to you both? Is there a reason mom did not include him in the first place on aNY of these documents? Do you generally agree about her medical issues?

You might ask her doctor if s/he thinks mom is competent enough to assign a new joint medical poa.

My brothers and I have joint medical poa, and while we generally agree, I most often end up deferring to my brother who is mom's sole durable poa, since he and my SIL do most of the visiting and heavy lifting.

Because he wants to be involved in her healthcare decisions since he takes my mother and me to her Dr visits.

My understanding of POA is that it can only be granted by the person themselves.

Why does your brother want to share Medical POA with you?