Hello Zamp and welcome to PC! What a tough situation. The only option I see is telling her flat out that your father's sex life has nothing to do with you. He's your father, you love him and that has nothing to do with her relationship with him or his with anyone else. Let her know that you love her, but certain topics are off limits.

If you're in a really sassy mood, remind her that SHE was the one who picked him to have a baby with.