jamreborn

I find it deeply disturbing that you mil doesn’t respect you and your wife’s wishes. I’m not sure how long your vacation is but depending on length you may want to adjust your plans. I couldn’t imagine how long it would take you to get back into a routine when you get home if it’s been upended like that.
If you haven’t already you may need to lay out the consequences of her not respecting your wishes like limiting contact.
She is not the parent and while she doesn’t have to agree with your decisions she does need to respect them. This matter really isn’t about harm to your child. A night or two of movies and sugar won’t undo all the healthy eating and activity you and your wife provide. Missing kindergarten also isn’t the end of the world. So perhaps, if you haven’t already, explain that it’s not about the sugar and lack of activity, and more about her not respecting your parenting decisions she might see your side.

Good luck. I hope you get your vacation.