That sounds really tough. I'm sorry your MIL is so hateful. I imagine she has a constant struggle with her own weight going on in her head, but that is
no excuse for ever fat shaming anyone. Can you limit your contact with her? I would want to start dealing with the issue now because I wouldn't want my baby to grow up around those kinds of attitudes. It's hard to stand up to someone who is essentially a bully. Are you able to just walk away from her when she fat-shames, or is the trigger already pulled for you by then?
Another thing you could try is drowning her with facts. Have you had a chance to look at [url=http://webiteback.com/forum/viewtopic.php?f=178&t=19207
this[/url] thread, or
http://webiteback.com/forum/viewtopic.php?f=178&t=18634this one? Poke around a bit in that forum, and maybe you can find something helpful.
I am wondering if you could practice something when you are alone as a response to her fat-shaming? It would be reinforcement and validation for you, and if you become quite practiced, you may find that you are eventually able to say those words in front of her.