OP, I’m so sorry this happened to you. It’s a real shock to the system to have to reevaluate a cherished relationship when you realise that person isn’t who you thought they were.
A lot of replies here seem to be focussed on the whole weight issue/discussion but the crux of this is about betrayal, about people you trusted being two-faced and unkind.
You say your MiL is easily led and will try to fit in with the prevailing opinion in the room. IME people like this, who are sort of social chameleons, are enormously insecure and can easily get dragged into playing along with anything (my DM was the same, bless her - she was a genuinely good, kind person, but so cripplingly afraid of causing offence or expressing a difference of opinion that she’d nod along and join in with almost any kind of shite to avoid putting her head above the parapet).
Maybe this was instigated by your SiL? Not that that makes it any better, of course, but it could mean your MiL’s contribution to the conversation was more about siding with and pleasing her daughter than any genuine malice towards you. But I realise that may be a stretch, especially if her remarks were particularly spiteful.
I think you do need to address it. Your relationship is now compromised anyway, so rather than suffer in silence and feel upset and anxious every time you see them, you need to let them know what you’ve seen.
I’d be inclined to let your DH raise it with his mother. She needs to be apologising and making this up to you. She should be the one feeling upset and anxious about seeing you. And if she’s not contrite - if she’s defensive or goes on the counter attack - well, then at least you know where you stand 