It's not uncommon for the WAS/WWS to blame you for all sorts of things that aren't on you--and even the mystery letter scenario is not infrequent. Cheaters cheat and often there are more wronged parties. This is STBXW's and AP's mess. What's your worry? Is unfair blame unusual? Any clear danger to your kids? You know you didn't do it--so clear conscience. You told MIL you didn't do it--people who trust you will believe you.
