Sorry you're dealing with this, Core.

It [censored]. There's no two ways about it.

Leave W to her chaos. Don't get involved in ridiculous ideas for co-birthdays.

I would say "we'll see what happens". Its non-committal and kind without being defensive or binding.
Remember - you cannot understand illogical thinking using logic.

Rise above. Be the light. Turn the other cheek. Be AMOAFWL.