I just got back from a weekend trip to the lake. This trip was actually well planned and communicated to W weeks in advance (180 for me).
We left Friday night - and on Wednesday before we left, W let me have it. Her tirade culminated in "I'm not going with you this weekend."

I'm not sure if I replied, but if I did it would have been, "I'm sorry, because we'll miss you." (another 180 for me)

W did come with us and Sunday we went boating.
some of W's comments: you're going too fast, stop splashing water, we're too far away from the dock....
W ended the ride with, "I'm glad you had fun, I'm never doing this again. Just give me a nice clean pool."

Monday, I told W I was taking the kids out boating again. "Fine, I'll just stay here and read my book". About 1/2 hour later D7 had to use restroom, so I docked and waited for her. She came running back from the lake house saying, "Mommy's coming!"

W didn't say a word until I rocked the boat a little too much climbing back in after jumping out and swimming a little with the kids. Then it was immediately, "You're going to tip us over. I'm never doing this again!" But we did have a peaceful evening and drive home.

Moral of the story: "Don't listen to ANYTHING your S says."

Merrick, IMHO, your W probably does need some time alone to think about what she will be missing, while she is missing it. You and the kids go to the beach house and enjoy - leave an open invitation to W to join you somehow. It's not a solution, but a start in the right direction.
I've sent W on trips while I stayed home, they just didn't seem to have any effect. W was just trying to get away - and me staying home had no affect on what she was getting away from.

I've had the conversations with my kids explaining that "mommy's mind and feelings are sick". The term "mentally ill" got me in big trouble, so I don't use it anymore.

plk