My DH usually likes to make the argument that they are older and can't take care of themselves very well, so they need our help. great, that's the excuse for THEIR behavior. and if you are expected to cook the meal, i vote that you set the meal. zip's suggestion of ordering a pre-cooked meal works well for me. if YOU will end up cooking, you can determine what you want to cook.
but what irritates me in your story is your DH. he's sitting around, wanting you to heat him up a snack and wait on him. oh no, i don't think so! do you do that at your home?
i had that situation with my own in-laws many many years ago. we were home visiting, we finished a meal, i stood up and started to help clear the table, and my DH started to hand me his plate and walk out of the kitchen. well, he helped clear and do the dishes at our home, he could darn well help at his parents' house, which i stated, very matter-of-factly. he helped with the dishes. his parents were shocked. i would be more angry at your DH taking you for granted than your in-laws. and that old folks thing? bull.
I couldn't agree more! The problem is not moreso with the inlaws as it is with the OP's dh. Her dh's worst offence is taking her for granted and allowing his parents and himself to treat her as a slave!
I think the MIL fakes being sick to get out of doing all the stuff, but to control the OP's holiday - aka to force them to come. Then WHAMMO - she's sick just before they arrive. Too convenient in my book!
OP - you and your dh need counselling. I suggest you print off your post and let the counsellor read it because it is extremely well written and describes how you are being treated by everyone.
You do not deserve to be treated as a slave by anyone, including your dh. If mine treated me that way, we'd be on the way to divorce court. He is turning into his father. Look at your FIL, if you let this continue, your DH will be exactly like him down the road.
Please go to counselling and if your dh won't go, you go yourself. Don't allow yourself to be a doormat any longer because you deserve to be treated like the wonderful and kind person you are.
Oh and for good measure, YOU should be dh's first priority, not his parents who love to guilt him and you to coming at their beck and call!