I didn't tell my fiance that his dad kissed me
Why wouldnt you tell your fiance that his dad kissed you?
I'm scared of getting myself in trouble so I wanted to hide it from him
This is not a great secret to hide from your fiance. If his dad really kissed you then this is something that you should tell him right away. The longer you wait the worse it will probably be.
I doubt he would never find out.
But if he finds out later down the line he's going to be so upset. I know it's difficult but I think you should tell your fiance about it.