HUGE stepsister problems?

It sounds as if you and your mom and stepdad need to have a talk and you need to let them know how much this situation bothers you. I have to admit that I would be really upset if I were in your place. Your only other alternative is to talk to Dave's daughter yourself and lay down the law (it may help if you and your sister do it together). Let her know that her ignorant, immature behavior is not going to be tolerated anymore and that if she wants to continue living in this house she had better shape up or ship out - tell her if she dislikes living there so much to go live with her mother and leave you all in peace. But the best course would be to talk to Dave and your mom and let them know that if this living arrangement is going to work out everyone needs to be respectful and pull their wait and treat each other with kindness and caring, no more selfish, childish acts. Good luck and God Bless.