Two of my uncles and my aunt have treating my mom, (who has POA) like absolute trash. They are always accusing her of wasting my Grandma's money, of not telling them things about her care, and not involving them in decisions, etc. I think she should flex that POA and refuse them access to my Grandma completely until they can apologize or she passes on. Whichever comes first. Can she legally do that? None of them are in a place to start or finance a legal battle.
Your grandmother has a right to contact with her children, is the point. If your mother has issues with her brothers, or if you have issues with your uncles, then that's between you; and I would encourage you to encourage your mother to express her feelings to them. But it is plain wrong to use your grandmother as some kind of bargaining chip to force an apology from them. Don't do it.
POA does not mean you get to determine who sees the grantor ( in most situations). Are Aunt and Uncles causing grandma agitation?
Has your mom set up a CaringBridge site to keep the family informed ?