If the food problems were caused by diabetes surely the sensible thing would have been to have a polite chat at the start of the weekend and explain her situation. If your host is unaware of your condition or how it might affect you then of course they will think you are rude. Failing that surely she could have said something like "I hope you don't mind me helping myself to more but my blood sugar levels are low"
And don't people with diabetes routinely carry some "emergncy rations", such as a Mars Bar, for this eventuality.
"Drew attention to herself" - well that could cover a multitude of situations but maybe, just maybe, she is one of those annoying people who insist on being the centre of attention all the time.
"Oversleeping" - again I think it would be a poor show to lie in until late morning when it is clear everyone else is getting up. You could be seriously inconveniencing any plans that have been made for the day.
As to the wealth, or otherwise, of her parents and the extravagance of the wedding - I think the point she was making was that Heidi had chosen a flamboyant and expensive wedding venue when neither she nor her parents had the means to pay for it. That's just daft! My parents didn't have much money and neither did I when I got married. But we had a small do in the local village hall with a buffet lunch - we didn't expect a huge party with someone else picking up the tab. It's called cutting your coat to suit the cloth.
I do agree that this MIL sounds like a bit of a bag. I really do. Bit I am afraid I don't like the sound of Heidi much either.