I think my mother is showing signs of some type of dementia. She has become cold, unfeeling and has angry outbursts regularly. Advice?

At 81 yrs of age she has been on Xanax 21 years since my sibling died. She takes 1 in the morning and 1 before bed. Before 6 pm she becomes unbearable sometimes. I have helped her painting, planting flowers, anything she needs me to do, but it's as though she does not remember in a week that I helped her with anything and she is starting to have some paranoia. Like she couldn't find her shampoo, so someone must have stolen it. She has a cell phone and thinks other people are taking it over and spying on her. Trying to tell her anything different means I'm calling her a liar or that I'm not believing her. My step-father has stayed in his bedroom the last 3 days because she has been getting so bad, yelling about every little thing in her house she thinks needs fixed, like a small spot on a wall. She has always had everything she ever wanted. She is never happy and saying cruel things to people doesn't bother her at all. She won't let me see her medicine or help with anything she considers too personal. We were always close, but she seems to be turning on me more and more. I'm her only daughter. My only sibling has his own problems and can't deal with her. If I question her on her medicine, she goes off on me. I'm getting so I just want to stay away, but then I feel guilty. When she says mean things to me it goes to my heart and I get depressed as well. I find myself crying more and more and not snapping back to myself after her cruel words. I see the same thing in my step-dad. He is always holding his head down and looks sad. Not sure what to do.