How to get my dad to believe me?

my parents have been divorced for over 6 yrs now and i lived with my mom up until a year ago.. my dad has been with his gf for almost three years now and her and her 3 kids live here with my dad and i.. she is a compulsive lier and just loves making my life a living hell when my dads not around but yet she convinces my father that im the problem. ive tried talking to my father about her behavior and he listens, but always takes her side and believes what she says. she has him on a leash and he doesnt see it. shes causing a lotta problems between my boyfriend and i, and shes trying to mess up everything. i dont know how i can get her to stop and just either stay outta my business or get out of my dads house. i know she makes my dad somewhat happy, and that he needs her there but shes using him for his money and making things a whole lot more difficult then they need to be... any suggestions? ideas? thoughts?