It sounds like it bothers her that she isn't spending holidays with her kids. I haven't read your previous threads to understand the entire history, but can you share why you didn't offer for the kids to be with W on Mother's Day. It might help to formulate the email response.
But something along the lines of:
I'm sorry it didn't work out for the kids to spend time with you on Mother's Day. I'm sure I would have felt the same had the situation been reversed.
And then as far as Memorial Day, it depends on whether you can make that switch to accomodate her or not. If you can, that would be a nice gesture.. just let you know it's okay and keep it short. If you can't, just matter-of-factly let her know that won't work, but here's what you can do.
