Can a caregiver/sister deprive or deny people or family from speaking or texting their parent?

Hello everyone, my name is Georgia and my sister is the caregiver for my mother. I am grateful she does it, but she has taken it out of control. She lets none of us talk to her on the phone, she has taken over all of my mother's social media,when we do call her she (my sister) gets upset and she threatens to block us because we're bothering her. So that is the question that I have. Is my sister allowed to deprive my mother from phone calls even if their regular phone calls, a hello?, A how are you doing? My sister wants to have control of everything. Today I called to say hello to my mother, and then I got a text message saying that I'm bothering her and that she was going to change her phone number. She (my sister) did not have any trouble receiving me when I was paying for everything but I became broke paying for it all and now I am a bother. I sent my sister money so that she can buy my mother a hospital bed and she bought an L shape sofa for her home. This is why I stopped sending her money. My mother always expects my phone calls.