What do you think about me telling my dad that I won't go to his house again unless his girlfriend and her children are gone?
That sounds serious, don't you like your dad's girlfriend and her kids?
I don't like them, they're using my dad for his money. She doesn't love him.
Do you have proof that she's using him for his money? If your dad is in love with her he may not listen to what you have to say about her unless you have proof to back your claims. If your dad is happy, then you have to accept his relationship.
How can you say that? You're wrong.
If your dad is happy then you need to be happy for him and try not to interfere. If you really do have proof that his girlfriend is using him then show him, but otherwise you have to accept this situation and make the best of it.