Let's look at the positive side of this. You and your SILs get along. They like you and your DH. They like the way you throw a party. They want to celebrate occasions with their brother.
All they are doing at this point is asking if there will be a party, and if so, when, so they can save the date. That's really not so bad. Right now, I'd respond, and tell her that nothing has been decided yet, but you'll let her know as soon as plans are made.
The real problem with these parties, as I see it, isn't the in-laws. It's your DH that is making them stressful. He starts big projects in the house just before the party. He's working in the kitchen while you are trying to cook. He doesn't communicate with you about when his part of the food is ready, so that you can get the other food out on time.
I'd sit down with DH and discuss the party. Do you want one this year? (Perfectly okay to have the party every year, every other year, decide on a lower-key party except for milestone birthdays, or not have a party at all.) If you are going to have a party, set the date.
Then discuss the problems with the previous parties--the projects, the project in the kitchen, the lack of communication. Tell him these need to be solved, as they are simply causing you too much stress, on top of the stress of having the party to begin with.
I wouldn't prevent him from working on a project the morning of the party, since that seems to be the best way to get something done around the house. But I would declare the kitchen off-limits. And I'd work out a plan or a system or something so that the two of you are on the same page as to what food gets served, and when.
My family acts the same way around lobster, clams, oysters and corn on the cob--joyous gluttony all around, a sort of tunnel vision, so I know the feeling.