This (using the evidence to expose, and re-expose, to family) is a great point. There's no legal threshhold with a MIL, lol.
I didn't want to overly harm, emotionally, my wife's parents, who at the time of my wife's affair were 85 years old. It was killing them enough as it was, without me playing/showing raw intel for them like their daughter having sex in her car with a man half her age, or openly talking to OM about LYING TO HER PARENTS about it, and how they needed to "just get over it."
I did, however, look them in the eye, and tell them their daughter was lying to them, that she WAS having an affair, including PA, and that "as painful as this is, Mom and Dad, I wouldn't be telling you this and upsetting you if I didn't have absolute proof of what I'm saying is true."
