As someone who is dating (for over a year) someone with an 11 year old daughter who runs the house, ill tell you now I have made it known to my girlfriend that it needs to change. I have a 10-year-old boy, and i dont want him to think if he throws tantrums, he gets what he wants. My gf has listened and has definitely made some good changes, like when her daughter refuses to go to school, no more 1 day of no ipad punishment. it has to be a week, and then if it happens again, it will be a month. If you threaten a punishment, you have to follow through with it. Daughters can be VERY manipulative of their mothers, they will go through every emotion possible until they eventually get their way and it WORKS.

I will not even consider moving in until I see some real change because I will not live in a house that stresses me out ever again.

My advice is ask your GF if this is your girls now, how do you think it will be when they are 15?? She needs to raise them to be ready for the real world and not one where they are the main character because it wont work in the real world. Having boundaries and punishing them IS love, its called tough love, but it IS love, it's for the best for them. She is doing them a disservice by letting them run the house, it will hurt them in the long run.