How do I get dad's girlfriend to quit lying about my boyfriend?

This is really weird…and I dont understand it. My dad’s girlfriend is really vendictive. Dad knows this and has admitted to it many times. He still continues to date her knowing she lies and cheats on him! I have tried to talk to him but nothing is working and I really need some help. My boyfriend moved in a couple months ago and stayed until last week. My dad’s girlfriend doesn’t want us to be together for some reason and I don’t know why. My dad doesn’t have a problem with him at all and she seems to hate him. She hates me too because she told me:( anyways here’s the thing. She is lying and saying that the police are going to bust him…and may I ask for what? Drugs?! I don’t think so lady! I have been with him 24-7 and he does nothing of the sort. She is trying to break us up because she continues to lie about it. He called the DEA of our county and asked him if he has heard anything about this and he said of course the obvious…NO! So for sure we know what she’s saying is not true.. What should we do? Im not eighteen yet but it is still hurting me and him. How do I convince dad that she is lying about the whole thing? Please help me-any help will be appreciated. Thank you.