My [40M] mother [67] intentionally ruined a long-planned and expensive Xmas surprise for my daughters [10 & 12].

I live in the US, my daughters live in England. I have been setting up a huge surprise visit the day after Xmas and I’m taking them to see Hamilton in London.
My wife, brother, mother and ex-wife knew about the plan. My mother told my brother about it yesterday, thinking he didn’t already know, and also told him it was a big secret and not to tell his nieces.
This morning I get an excited FaceTime from my daughters about how they can’t wait to see me and go to Hamilton. They told me that Grandma told them this morning.
I confronted my mother by text, first she said that she didn’t realize it was a secret. Then she changed her story to say that she thought that Hamilton was the only part that was a secret (which she also leaked).
1) What would motivate a person to do this? 2) How do I get over this crushing disappointment? 3) Do I make her explain her thinking to me or just let it go?
She has either become evil or senile overnight.
TL;DR: my mom ruined Xmas.


Source.

Crushing disappointment? Geez, lighten up and take some Pamprin, it's obvious your vagina is bleeding.