Help...mother has become difficult and mean and is affecting my father's well being and putting a strain on our family.

I live at home with my 70 year old parents (I'm 40 and single due to my obligations here that make it difficult for me to have a life of my own). My mother retired a few years ago and has become increasingly worse since. Her and my father were everyone's ideal parents. They had a good time together. My dad has always been more social than my mother but was never a big issue. Now my father can't even go outside in the front yard without my mother accusing him of staring at our neighbor (woman) across the street. He feels like a prisoner and has shown a decline in health because of this. I believe my mother has the beginning stages of Parkinson's. She is very suspicious of everything and plays the victim when shes the one that causes all the drama. My father has repeatedly told her that there is nothing going on with the neighbor to the point that he just doesn't acknowledge her accusations anymore. Shes gone as far as confronting the neighbor where she also told her that she has nothing to do with him...they dont even speak. But somehow she has put in her head that my 70 yr old father, with bad knees, sneaks out in the middle of the night to visit her. And that they have some sort of good night signals that they give. Its horrible. My father wants to leave but she always apologizes. Only to start it all again the following day. She gets upset when I tell her anything about it. She right away says that my brothers and I side and will cover for my father. She plays victim, starts crying and will throw out either "I'll leave so you can all be happy", or "sell the house so I can leave". Then its silence until she needs something from me. This has not only affected my ability to have a life of my own because they've become totally dependent on me for everything (they are still active and able to walk but have me run all their errands), and give me a hard time when I leave the house (guilt trips are given almost daily here), but she has also created friction with my brothers that they dont come over often just so they dont have to deal with the drama. Which again leaves me with the burden daily. I dont know what to do. I want to leave this house because it feels so toxic being here but I dont want to just leave my father. I'm also afraid of how my mother would handle it if she was alone knowing she was the reason for all this drama. She likes to say hurtful things to my father, but plays the victim when confronted. She refuses to get evaluated to see if its something that's making her like this. If that was the case, we'd be more educated on how to deal with it properly and know that this behavior isnt just her being a jerk to everyone...its actually a disorder. Any advice would be greatly appreciated. Sorry for rambling but this is wearing me out.