This is how Celestia, Luna, and Cadance learn of Twilight's betrayal goes in My Little Pony: The Revenge of Nightmare Moon.
[A guard runs into the throne room]
Guard: Your highnesses, something terrible has happened.
Princess Celestia: What is it?
Princess Luna: Has Thunderwing come back for revenge?
Princess Cadance: Has Unicron sent another servant to wreak havoc on Equestria?
Guard: I regret to inform that Twilight Sparkle has been put under Nightmare Moon's control.
[Princess Celestia, Princess Luna, and Princess Cadance gasp]