What are the rights of adult grandchildren, who do not have POA, when the POA purposely discludes them?

A number of years ago my grandmother began having major memory problems which spiralled into what was eventually diagnosed as dementia. At the time I was barely a teenager and the circumstances and symptoms were annoyances rather than worries (trying to cut up food, treating me like a very young child...my parents and I had lived with her for the first few years of my life so it simply seemed she wanted to return to that). My mother, my grandmother's favorite child, died during my preteen years. I don't know whether she had POA for my grandmother. (My mother's parents had long been not-so-amicably divorced by the time of my mother's death so I know it was not my maternal grandfather.)

After my mother's death, my uncle - my mother's younger brother - was thrust into being the primary caregiver for both his parents. He's nowhere near as competent at diffusing tension as my mother, and that has not helped. My father was unable - and, I suspect, unwilling - to care for either of my maternal grandparents, a fact that my uncle did not 'understand' or accept. He also believed that I needed to take more responsibility in communicating with my aging grandparents, but I was easily exhausted by talking to them and my father refused to discipline me on the matter since they were not his parents.

My grandfather then died, about two years after my mother. He'd not changed his will after my mother's death, for whatever reason, so his financial assets were divided in thirds: his girlfriend, my uncle, and my mother - who had specified in her own will that any assets of hers were to, by default, be mine. My uncle had recently lost his job, and in managing his father's assets managed to squander much of what would have been mine since he was angry about the fact that a 14yo would 'have' that inheritance. (Which was then in my father's name until I was 18; now it is mine.)

My grandmother has always been skinny, and during my high school years her physical health swiftly declined, and her mental health only slightly more slowly. After a couple of falls in her condo my uncle coerced her into moving from Michigan to Florida to live with him and his wife. (She could not care for herself, but would not agree to a nursing home; that was their compromise.)

We stayed somewhat in touch after that, but I went to boarding school and did not see or talk to them as much as they wanted, which only made my uncle more bitter, which I noticed even when visiting. I wrote it off as stress - after all, I was a high school senior, and knew all about that.

Over the course of another year, calls I make to their house - for a while, more than weekly; now, perhaps once a month - went from very brief calls to the phone going unanswered.

My uncle now refuses to even speak with most of my grandmother's friends - many of whom are women who are on the brinks of their own deaths, and a couple of whom helped raise him and my mother!

I eventually sought my father's help, and he managed to reach one friend of my grandmother's, who with struggle got my uncle to say that he'd admitted my grandmother to a nursing home. He did not tell her the name or where it was, and responds to no one at all, at least no one whose phone number my father or I have. I still occasionally call my uncle's home phone; the voicemail is the same as it's been since I was a child, which means, presumably, that he and my aunt are still living there.

Most of my grandmother's friends and their caregivers are not willing to get into an argument with my uncle. He's a funny guy but is irrational, narcissistic, and aggressive. As much as I do not want an argument either, he and my grandmother are relatives who are important to me and we are the only close biological family of my mother's who are still alive, and I am now an adult, who is determined to have at least some semblance of an adult relationship with them, at the very least with my grandmother before she dies. If nothing else, I would like to see her one last time.

However, I don't know what right I have to challenge my uncle. He has POA and at this point I doubt my grandmother has much, if any, lucidity or sanity. I do not know where she lives and cannot even find my aunt at her former place of employment.

Does my uncle's POA give him total control over whether I am privy to information about my grandmother, and whether I can see her?