So my fiance and I found out last night from his high school friend that my fiances dad (lets call him R) so R has been cheating on my fiances step mum for atleast a year. The high school friend had never met my fiances dad while they where in high school but this friend knew that R was engaged and where he lived. So the friends flatmate was apparently sleeping with R and this girl and R would frequently be over at this friends flat to hook up with the flatmate.

So we know its true. And the thing for us is that we dont know what to do.

I do not get along with the step mum as she has made many passes at me in regards to my kids (she doesnt have any kids and told me she would steal mine while I slept) thats not even half of the sh1t thats happened. I may not like her but I know if it was me Id want to be told.

My fiances view point is that his dad will find a way to make the truth look like its lies and then get the rest of the family (the ones I actually like) to turn against us like he has tried to do before when I was having issues with the step mum.

We are both at a loss with what to do. I thought of somehow bringing the girl he cheated withs name up and watch his reaction but R is a good lier. The step mum even suspected he was cheating 3 years ago and told me because R wont even let her use his phone to check the time and wont give her his phone pin and she used to always get jealous how my fiance didnt hide his phone from me ever.

What would you do if you where in our situation?

We dont want to cause a big family fight.