Do I have the right review mom's documents even though I'm not POA?

ive been poa and ive been carer without poa . i dont think sharing and being on the level with everyone can be anything but beneficial .
your statements back up each other . at some point if your words are consistant , people have no reason to distrust you .

Quite the opposite, the POA is supposed to keep personal information confidential.

Why do you feel you NEED to review the documents? I sense there are some issues of trust either between you and your brother or with your mother.

Read this.. It doesn't answer question but is informative
https://www.agingcare.com/articles/family-fights-over-power-of-attorney-153319.htm