My sister has POA of my mother and I am the youngest (also I was adopted) sibling, my sister always threatens me on not seeing my mother. Any advice?

(My mother has dementia and her memory is not as good, but she ALWAYS KNOWS who I am and we are inseparable) My sister has this idea that she tells me when, what time, and how I can see my mother (if I haven't told you I am 52yrs. Old and my sister feels cause she is 67 yrs, she has this right to call the shots). I really feel that this is just another form of her way to control me. I really need advise on this. I never abused my mother, lived with her and took care of her every day for 10 yrs. Never asking my 3 siblings for help... I finally took my mother advise in 2013 and married my best friend. This completely made my sister lose her mind even more..!!! I just want to spend time with mom she is 91yrs. old and I know there is not a lot of time left to see her, SOMEONE PLEASE HELP ME.!!! JUST WANT TO SEE MOM.!!!!!