The nanny is not explicitly stated to be a witch.
The book does have supernatural elements, but they're presented in a way that leaves the reader wondering if they're real or simply the protagonist's imagination.
The nanny is a mysterious figure with a strange past, but her true nature is never definitively revealed. The ambiguous nature of the nanny's powers contributes to the book's suspense and unsettling atmosphere.
It's up to the reader to decide if they believe she possesses supernatural abilities or if she's simply a very skilled and manipulative woman.