Stepdaughter does not behave

Im writting because I just don't know what to do with a big problem im having. I recently got divorced this past March. I have a 7 yo son. I make a point of calling my son 3 times a day and to go see him as much as I can. The divorce has been very hard on my son. I've been living with my girlfriend and her 8 yo daughter for 9 months now. I love them both very much and I consider her daughter to be my own. In the begining the daughter hated me. She would always ask when I was gonna move out or she would tell her mom to make me leave. Mom would always back me up and tell her daughter that I wasn't going anywhere. About a month into living here the daughter warmed up to me and we were the best of friends. My problem is that my girlfriend doesn't discipline her daughter when she gets out of hand. This little girl you would think is the devil. The daughter pretty much runs the show at home. The daugter won't let my son come to my home or do sleep overs. My son has only met her once but she refuses to give my son a chance. This has really taken a toll on my son. Shes very nasty to her mom and myself. Mom has to ask several times for her daughter to do something. The daughters responce is always in a min. Most times the daughter will be nasty to me and mom will tell her not to talk to me like that; the daughter will tell her mom to bad ... I just did and theres nothing you can do about it. The daughter can't go to the bathroom by herself ... mom has to wipe her butt all the time for her (this girl is 8) She can't take a shower by herself ... mom has to be in the bathroom with her. My biggest issue is that the daughter won't sleep in her own bed ... instead my girlfriend lets her sleep on the floor next to our bed for the sake of arguing with her daughter. If the daughter does happen to sleep in her own bed (very rare) like clock work around 2 or 3 am the daughter will scream ( MOM ... GET IN HERE NOW! ) and sure enough mom goes in the daughters room and sleeps with her for the rest of the night. This little girl knows how to manipulate mom to get her way and if she doesn't the the tears start flowing. I've tried to talk to mom and tell here that when the daughter is bad she needs to be disiplined. Take something away from her when shes bad. Her daughter goes to dance classes 4 times a week. I've told my girlfriend to take dance away for a week when shes bad and mom says I can't do that to her. Ive even went as far as offering to go to parenting classes with my girlfriend. I told her I would go with her and that I wouldn't let her go through them alone. The only answer I got was that she'll think about it. What do I do? I love these two alot and don't wanna lose them but life is very hard for me around here because of her daughter. I've often thought about moving out as much as I don't want to.