My non-guardian niece will not give me address or phone number to speak with my Mom. What do I do?

My mother has lived with my sisters on and off back and fourth in Florida for years since her diagnosis of dementia. This is a very long and sad story But the bottom is that I have always went along with everything and everyone even if I knew the truth of what my family has done, just so that my mother can be happy for a just a little while. but since my sisters mother in law passed they now do not want the burden of taken care of mom, (her husband does not like mom-never has) and the oldest is always making excuses not to take her in as well. So they passed her on to live with a niece in VA, They didn't even ask me to see if I wanted to take care of. But then I will be honest I would love more than anything in the world as to be able to stay home and care for her but I cant afford to and mom needs 24 hour attention. So I agreed at the time because she would be very close to me I am in NC, that way I can see her more. but as soon as I said yes I requested no problems and no more issues, because my niece - by the way never called nor visit my mother (her grandmother) in years 5+, then all the sudden she will take care of her, anyway she and I had a misunderstanding and she is still upset about that. and will not give me the phone number or the address at to the where about of my mother. My sister's including the mother of my niece are just dragging it on and will not help "they got rid of mom: now I have to put up with this garbage. All I want is a phone number to call her when I want to and see her without an issue. I am tired of the family drama and only want peace for my mother and to enjoy her as much as possible. What can I do to get a phone number and address to see my mother. She's making me go thru a 3rd party. Help.
My sister tricked my mother (who speaks no English) to signing over power of attorney before she was sick (another long story) and gave her to our niece who had no power of attorney. this power of attorney and all the issues and decisions that were made for my mother where made by her and ant one point one of the brothers without the knowledge and or even asking any other of the siblings, there are 6 of us and four of us never knew what they were up to at any time. I don't care about the past nor do I care of the money and manipulation and what they did to mom, I just want to hold her and love her and be her daughter without any issues. please someone just tell me what I can do. I am not a rich women and cant afford an attorney, and they know that.