Fiance Flirting With My Father

My fiance is always quite openly flirty with my father. They always greet with a longer than normal embrace. I have noticed his hand on the top part of her ass when they hug.
Yesterday when we were leaving their place, as I left the front door and turned around, I saw her hand slowly trailing across his back. I don't think she expected me to look.

I absolutely enjoy every moment of this and try to gently encourage her without making it obvious. I want to find a way to let him see some of her nudes on my phone soon.

I wish I could just be open about it and us all have a good time with it, however far she felt like taking it.