Holy shit! Your mother and in-laws are whacked in the fuggin' head!!!! Seeing as how these ass-clowns are still your blood relatives, I'd take my ass right on over there, and call them right out on their "inappropriate, disgusting behaviour!!!" They'd be given a dressing down from me like they've never had in their lives!! Who da fuq do these people think they are? I'd seriously bitch-slap someone.
And they wonder why people are leaving?
The silver lining to all of this? #1: Your ex-wife is really getting a good dose of their "wholesome Christian love", and I'm sure the cognitive dissonance is building up in her. Wow! #2: Your daughter won't forget this episode, EVER! It will stick in her brain for life. I know, similar shit happened to me as a kid, even though my parents were in good standing! They were nearly poor, no position in Org, etc, so I never got invited and was excluded from everything.
PS: Fuck these assholes! SMFH.