Hello. Welcome to PC. You're a good man. Keep treating her daughter like a daughter and being supportive. Have you had a serious conversation with your girlfriend about this? She presents a negative picture in your depiction. Her daughter will eventually get to the proverbial last straw and the relationship may be broken for good. What about mom and daughter going to therapy together? Probably not a great idea but if push comes to shove you could get them together and act as a mediator (in lieu of counseling if they refuse). Good luck.