Help-my dads girlfriend is a psycho?

I have a problem. My dads girlfriend treats me like crap. It's been going on for ten years. She is racist and has extreme Nazi type political beliefs and we used to fight alot about it as I believe the opposite. She also puts me down and says condescending things to me. An example-we were discussing Will and Kates wedding. She carried on about her. She then said how poised and elegant she is and carries herself well. I harmlessly said-"she might have been trained." My dads girlfriend hit the roof (in public at dinner) saying "what a load of utter bullshit," and tossed her head back and went tut, tut etc. At MY dads place she acts like she owns it! She controls the remote on my rare visits and puts on the worst shows. When the news is on there's constant racist etc comments. My dad said for me to think of the house as mine, and I have a key, but how can I when I feel so miserable there?? She won't let me touch anything in the house. Once I went to get milk out if the fridge and spilt a drop. She screamed at me "Oh my God, what the he'll happened here? Clean it up and don't ever touch anything again." I hate our arguments so have had to only see my dad twice a year. I get pushed by her to leave after only one night, despite her family staying there for months during the year. My dad always defends her and blames me for the problems. He said "you nearly caused us to break up." She has her own house and I suggested to dad she go there when I come so we can have time together. He says not to cause trouble and she is nit going anywhere. She seems to get nasty after a drink of wine! She also told that a child (I'm in my 20's) should not spend any time doing things with their parents. My mums dead for a while and I only have dad. She also brings up stuff like the cost of my meal dad has spent on me when he cooks for her, provides accommodation, drives her around and takes her for dinner every night of the week and pays for food. I hate that as my dad gave me little aide.