Goodness, at different times of my life I would have responded to this in so many different ways... and I really don't think any of them would be wrong - but they would all lead to different consequences. If you see what I mean.
Now, in
this decade, at
this age, being who I am NOW with the knowledge I have NOW, I would stand up to her, politely and implacably.
She tries to control people with food?
Stand firm and reject that control.
Polite.
Friendly.
Absolutely non compromising.
'Oh, thank you Ma, hubby said you were having fish and chips, but being diabetic, I can't eat those or it will make me ill. so I have brought a (insert whatever) for me to eat.'
Then not one chip, or fragment of batter, or desert spoonful or biscuit can pass your lips.
Rejecting her food will send her into a complete spin.
There will be further games but the more control you give her, the more she will want to take. So every interaction (?skirmish?) sets the tone for the future. And it may be a lifetime campaign.
I just don't want to be controlled by people who are silly and selfish enough to not respect my future health.
But then, I really am a B!tch.