I used to have the same problem with my family (aunt to be exact). She knew I was trying to eat healthier and I would ask for things to not be brought and I was given the support, at that moment. She was the one who would constantly make snide comments so you would think she would be supportive. However... when she would visit, she would bring the worst things, including dessert, and then get offended when I declined what she made. I was ungrateful, I didn't like her food etc. She turned my goal into her being a horrible cook and person!
I think your MIL was maybe testing you? (Makes me wonder if I was being tested, was I being set up to see if I would give in?) I don't think you're exaggerating. I see the timing as odd. Maybe I'm projecting? lol
Hang in there. Keep making the healthy choices for you and let her/them see you mean it. After all, this about you and your health, not her/them.