I found out my mil is having an affair
Wow did you catch them? Does that bother you a lot?
Yes I'm interested in this to the extent that I have an imagination about them fucking.
This affair is her problem, not yours so do you really want to cause drama? Keep it to your imagination like you said.
Instead I'm going to take this and haunt her over something she did. I don't like how she acts around me and it's payback time.
You should stay out of other people issues. You could end up hurting the entire family and possibly ruin the relationship.