Sometimes these things can be a blessing. I understand that you might be insulted by their decisions and you are probably insightfull enough to come to the conclusion that they are not staying with you because of their judgements about you (which may be wrong, wrong, wrong). That alone is insulting.
On that note, let's just think things through a little bit more. What's the worst case scenario?
... they judge you harshly as someone who isn't worth a damn. ~ insulting!
... they think you suck. ~ insulting!
... they have opinions about you that are NOT TRUE but there is nothing you can do about it to change their minds because they are bent on pre-judging you how they wish, based on nothing... almost seeming as if they enjoy looking down on others because it just plain makes them feel better about themselves. ~ well that's just sad!
I say, 'So be it!' and 'So what!' about their choice to stay elsewhere and I say '~whatever~' about their judgements. Not being around that toxic emotion means all the less worry and anxiety for you. If "people pleasing" is a temptation for you, a struggle for you, within your sweet heart... than you can consider yourself spared from the intense observation.
You have been given a lot of breathing room now. And in that breathing room, you can keep focused on who you truely are, you can remind yourself that they won't be here forever, your anxiety will decrease, and you can have your own home and live in it as you see fit. And if they don't like it they can go back to the other house.
My only tender advice is this: Stay true to yourself. After they leave, if you stay true to yourself, you will feel so good. Just be welcoming, pleasant, and a calm relaxed host when they are there. Have an aire about you of peace, carefreeness, and represent that nurturing side of yourself well with an attitude that everything is going to be alright.
And honestly, take a Prozac (or whatever other anti-anxiety medicine your doctor can prescribe) if you need to give yourself a break from struggling to maintain or cope through it. These things are perfectly OK you know.
If they bring their own food (my neighbor for example eats only organic -labeled- food) cook it up for them, or let them cook it themselves. Be a servant type help to them, getting out the pots and pans they need, asking them what you can do to support their fine cooking time. And praise them for being so smart about food, for learning the food facts that they may share with you (to try and educate you), and tell them how nice it is to actually have such wonderful cheffery in your kitchen. What it a treat it is for you to step aside...
The silliness or the reasons why they do it, don't matter. Don't be insulted. It's really not your issue or probably not meant to be a hint about how uneducated your food IQ is or how unskilled your cooking skills are.
"Food issues" are an obstacle too big for you to deal with. Your job is not to change them, only to love your husband, their grandchild, and smile sweetly.
And when they go just take a big breathe in that moment and let it go... It will be over and you can give yourself a big pat on the back for not letting anyone (or any judgement) creep into your heart causing you great grief.
What a great example of integrity you can be to your husband and daugther... that's the cleanliness that counts. A clean and clear heart. Everything else just falls short compared to that and is almost unworthy of any further thought or consideration.
It really is the HEART that makes the home.
God bless, YM