Hah, I see people talking about 'A Twisted Tale' as if it's a single novel when it's actually a whole Disney-themed anthology series where each book asks a 'what if' scenario. So there's no ONE main twist across them all, but the whole premise is the twist itself. Like, what if Ariel never defeated Ursula? What if Mulan had to travel to the Underworld? The twist is always taking the familiar Disney movie ending and subverting it completely, usually in a darker direction.
That said, if I had to pick the most iconic twist from any of them, I'd point to 'Part of Your World' from 'What If Ariel Had Never Defeated Ursula?' That whole book flips the happy ending on its head—Ariel loses, Eric is turned to sea foam, and Ursula rules Atlantica. The twist isn't just that she loses, but how she has to live with the consequences and find a way to fight back from a position of utter defeat. It's less a 'gotcha' moment and more a sustained atmosphere of 'oh no, everything went wrong.'
My niece hated it because it was too sad, but I found it refreshing. The real plot twist is that these 'happy ever after' stories have teeth when you pull the safety net away.