That sounds like a mess!! I honestly think that mother-in-laws are given a manual on how to be the most psychotic people on the plantet. When I was engaged to my husband, my in-laws FREAKED OUT. I mean like... should have been sent to a psychiatist freaked out. Started calling me a wh@#$, trying to diagnose me with mental disorders *Rolling eyes* so we eloped and didn't tell ANYONE expect my mom and dad. Who I am very close to. They found out and went even MORE insane. Saying they were disowning him, etc. Of course... they didn't and they showed up (My husband is from another country, so I THOUGHT we would be safe hahaha) and didn't leave for a MONTH. To make matters worse, they came when he had to leave for a week long conference for his profession and I WAS ALONE WITH THEM!!! OMG Talk about HELL... she did everything but kill me with that knife she was shoving in my back. Then to add insult to injury, she offered to PAY ME to divorce her son. OMG
Moral of the story... Well, there is none other than in-laws suck. (No offense to all the in-laws out there! I am sure there are good ones out there. My mom and dad is to my husband. Heck, they side with him on every argument that we have!!!)
My advice, let your fiance do the talking to them. I know he probably doesn't want to deal with it but it will save you looking like someone trying to break a "mother/son bond"... even if it is non-existent!