How to help girlfriend with her authoritarian parents?

My girlfriend and I are absolutely perfect. Our relationship is better than any relationship I've ever heard of. It is equal, we give just as much as we take, we talk instead of fight, and other people tell us we are so cute :) I am so excited to spend the rest of my life with her when that time comes. But there is only one problem: her parents. They have no problem with me (as far as I can tell anyway) but they are absolutely horrible to her. They always yell at her, blame everything on her, and they never allow her to make her own choices, even when they seem to let her sometimes. She is 18 now (but she still has a year left in high school) and should be making her own decisions. Here is some background into their family. Her parents are around 60, and she has a brother and sister, both around 30, so she is surprisingly young compared to most families. Her mother is a teacher and is about to retire to avoid being laid off. Her dad is a retired cop and was a pilot in the Vietnam war (I believe that much of this problem was caused by any trauma he may have suffered in his life). Her sister is married and is very successful even though their family is relatively lower class. Her brother is recently divorced and lost almost everything but his 2 kids, so he is very dependent on their parents for a lot of things. Both of her siblings are pretty smart but not geniuses, but she is below average, and has trouble in school with ADD and dyslexia, and feels very overshadowed. Sometimes her parents call her stupid and insult her. According to her, her father used to beat her and her mom (but doesnt anymore) and now her mom acts like her dad because she is afraid of what he will do to her. I could go on for hours about the things her parents do to her, but the main thing I am worried about is what this is doing to her. She actually blames herself for her parents' behavior sometimes, and she actually believes some of the horrible things they tell her. She has horrible self esteem, and I think this has contributed positively to they way she treats others, but every attempt I've had at contributing to the way she feels about herself hasn't worked. I do believe she has improved ever since we got together almost a year and a half ago, but I don't know what to do about her parents. It hurts me so much when I see what they are doing to her, and I can't find any other options. There's no way her parents would pay for her to see a therapist, she cannot see her school counselor because she is apparently cold and heartless, and the worst part is that she cannot actually talk to her parents because they don't emotionally connect and will take any sign of disturbance of the status quo as intending to "attack" them. They are impossible, and I care about her so much, but I don't have anything left to do. Can someone please help me!!!! :(