My girlfriend has a 15 year old daughter (I’m 49). I enjoy her company a lot. She’s a great kid. The thought of any kind of romantic or sexual relationship with her turns my stomach. It should also turn your dad’s stomach, and he shouldn’t be allowed to get away with “I enjoy her company”. But that’s just my opinion. I don’t know your family dynamics, and I know it’s easy to preach about what I think should be done from the outside.
Very tough spot to be in, and I have no advice beyond confrontation. I’m sorry you have to deal with this.
Or you wake up and there’s nothing there because that star died a thousand years ago and the light from the supernova hasn’t reached earth yet.