My Dad had a stroke. My brother got POA to do his property taxes. My mother wants it back.

So if dad is able, he revokes it and does a new one authorizing someone else.

POA is not a piece of paper that entitles the bearer special privileges, it is a legal document signed and witnessed whereby one person (your father) grants the authority to their appointed attorney (your brother) to make medical or financial decisions when they are not able to. If your mother is named as primary and your brother is acting as secondary, or if the POA has provisions for more than one attorney to act together or separately (jointly and severally) your mother's POA is still in force and she already has the authority to act. Often the lawyer who drew up the document will have a copy.