This is how Twilight's friends warn Princess Celestia and Princess Luna about Twilight goes in The Return of Thunderwing.
Rainbow Dash: Princess Luna! Princess Celestia!
Princess Celestia: What is wrong?
Pinkie Pie: Thunderwing's back!
Starlight Glimmer: And he's turned Twilight against her friends.
Trixie: We must get Twilight.
Princess Luna: Then we must alert Optimus Prime immediately.
Princess Celestia: Indeed, Sister.