You poor thing!! (Hey, we have the same birthday)
I know just how you feel. My judgemental MIL knows everything, and just out of principle, disagrees with everything I say. ("I do not!"--lol) My husband recently had knee surgery, so I was stuck spending an afternoon with her at a hospital an hour away. I wasn't going to mention anything until I looked like I lost weight, but she offered to buy lunch or ice cream, so I felt I should say something. (I gained 70 pounds since I married her son almost 10 years ago--I'd never been over 150 or so unless I was pregnant--2nd marriage, 2 kids from previous, I have a big mouth--tons of black marks against me!!)

I told her I gave up sugar a couple of months before. (That got me a sincere "good for you". ) I elaborated some, and said I gave up junk food. ("that's WONDERFUL") I told her how great I feel, that I have tons of energy...that I an eating only healthy foods, that I refuse to put anything REFINED into my body. I even mentioned refined flour, specifically. By the time I was done, she was practically cheering for me. I had just watched her eat a roll with her caesar salad, and she even made some feeble comment about how she knew that whole wheat would have been better.

But I never said "Atkins" or "low carb". She's way too conservative in her thinking to handle that. If I did, she would have responded with the usual "oh, Patty", with all the disgust she could muster!! (If I had a nickel for every time....lol)

BUT--she also hasn't SEEN me eat, and all that I can put away. And it sounds like your MIL will be watching you like a hawk. I think there is a low carb book about 'the diabetic' diet, or something like that, that you could mention. Or just tell her the doctor SAID to do this. (Mine approves) Or, just expect to have to grit your teeth and endure her AMA rhetoric, and eat what you want. (Bring a BIG roast...and eat pork rinds in front of the SIL...)

My dh can't stand up to his mother, either, so it makes it twice as hard. I'd say be armed with all the information you can, and stand your ground. And when you are saying good bye, tell her to "eat $xxt and bark at the moon!!"

Good luck--and keep us posted!!

Patty