I feel now I must share my sort of shame.
Mentioned in an earlier post (can't remember which thread) that part of the reason I've been so anti-MLM all this time is as a result of working in Fraud Prevention for the best part of a decade, specifically in the recruitment sector, where Herbalife / Forever Living / Younique and all the rest are collectively banned (at least by any reputable recruiter...), but never cease in their constant attempts to find new victims!
Perhaps I should thank them and their ilk for keeping me in work... 
Until recently, I'd never have heard of it otherwise.
But my DStepSiL has now become "one of them" (HL), with all the usual FB self-aggrandising paraphernalia that entails...
Knowing what I know and tipsy, I did challenge her on the true nature of MLM on her FB posts, and it all ended up a bit of slanging match - not swearing or insults or anything like that, but apparently I was "being negative about a business I don't understand". If only she knew! I was pointing out the inherent flaws of the "business model" - she took it as a personal insult.
I am usually very conflict-averse, and have never ever had a FB tussle, or ever fallen out with a family member - step or otherwise.
Eventually I realised this was probably about as productive as trying to de-convert a die-hard Scientologist, and STFU for the sake of future family harmony (my lovely mum in particular would be very upset if there were discord between us - though she has a few MLM-bots of her own, and thinks they are "barmy").
It still leaves a nasty taste in my month, a good month or two on. I doubt the relationship with my little step-nephews (1 and 3) will ever be allowed to be the same as a result. We are all pretending to get on for the sake of the "faaaaaaamily", but at the same time seem to sense tension on both sides. Fortunately there have been no face-to-face meet-ups in the meantime, as that would have ended up very awkward...
She is a lovely girl, a lovely person, and a lovely mum. Breaks my heart to see her fall for this bullshit, and am only surprised that her otherwise sensible husband is fervently supporting her in this crap....