MIL obsessed with weight

No you’re not overreacting at all and your husband should be supporting you. You should either have a quiet word with her, or write her a letter (not mean, polite) to explain your history and request that she does not discuss these topics in front of you again.

All of it above plus not in front of your child, ever. I have a very bad relationship with food and I'm trying not to make it an issue for my son, so for me this would also be a very sensitive issue.
Definitely try and talk to her especially if she doesn't mean it in a bad way but just has literally nothing else to say but if you've talked to her, told her how it makes you feel etc and she ignores it and still does it then I'd say you need to a have a conversation about boundaries with your partner and then you both with your MIL.

Good luck!!

My mum has a very bad relationship with food. She has severe diabetes and still believes she can lose weight by starving herself. As a child I was constantly on a diet. Even when my children were little she mentioned over feeding them by giving them too much milk. With my children I’m not focussing on what they can and can’t eat as I believe that does more damage. I tend to ignore any advice from my mum. Her eating habits are horrendous like putting cream in cereal. I think just have a standard response like you don’t want to talk about weight in front of your child as you don’t want to create issues especially being a girl and focus more on being active and generally try and shut down that topic of conversation. It’s probably and easy go to conversation starter for her which doesn’t help.

Next time try saying “it makes me uncomfortable and stressed when people talk about dieting. I apologize for not mentioning it sooner because I know it’s a part of your life that interests you, but maybe we could talk about XYZ instead? I heard you have a good story about that”

I had a similar issue with my mother who with my childhood caused an unhealthy relationship with food, my image and my body, some significantbody dysmorphia. I drew a hard line that its ok to be proud of maintain8ng or losing weight but in my home and especially around my daughters there will be no talk of "that food is bad" "oh im so bad for eating this" or anything else food or body image related, and that there would also be no comentary on ANYONES body. Its a hard boundary for me, i wont have my girls growing up thinking they are less than because of their size, whatever that may be. I model healthy mobement and self love in my home so im hoping my girls will see and not have the issues i do.

Send her fun articles or youtube videos on body dismorphia and the extreme negative psychological effects of counting calories or how a lack of calories causes brain damage and can lead to hair and teeth loss, weight gain and dementia related disorders if she's obsessed send her the negative reviews from doctors and health experts on the weight watchers program fuck her shit up