There are so many MIL (or other In Law) threads on here, over and over. Different stories but also all the same. I find them facinating reads as I actually get along with mine!
The advice from most readers is always the same - talk to her/him/them and if they don't stop/start whatever it is they are doing, tell her/him/them to FOff, stop seeing them or from them seeing the GC etc.
The thing is, nobody ever comes back and says whether they took the advice and what happened.
PLEASE PLEASE PLEASE, for my own morbid curiosity, have any of you out there actually taken the hard line approach with the In laws from hell? And what has the outcome been?