Not quite the same I know, but we were invited for a meal to the home of another couple years ago. When we got there...there were two more, a mother and daughter ( grown up), I think. We knew them from church, but had never really taken to the whole family, so the evening was spoiled for us.
At least you have been forewarned, but a holiday is a much bigger deal than one meal. I think you should have been asked first though, before she was invited. However, it will be harder for her I would imagine, not knowing the rest of you.
I do hope you still get to enjoy your break.