You are just talking about a twist ending, there is nothing wrong with it.
Consider The Sixth Sense, with Bruce Willis. A wildly successful twist ending, to be sure.
In the end, it turns out Bruce Willis was dead the whole time!
In your story, in the end, Olivia is really the legendary, fearsome, master baker Beatrice! Cue up gasps of astonishment.
The secret to a good twist is that when the reader discovers that Olivia is really Beatrice, and returns to the beginning looking for Olivia scenes and Beatrice scenes, you (the author) did not LIE to them, and you left them some clues, no matter how obscure, that might have let them guess this. Nothing Olivia does is inconsistent with her being Beatrice (or Helena). Olivia is never in London when the story says Beatrice is most definitely in Cork, for example.
Using The Sixth Sense as an example, I did not see the twist coming (even though on first watching I knew there was a killer twist!), but I watched from the beginning immediately, and sure enough, there were clues, and the movie never lied to the audience. Bruce Willis did nothing inconsistent with the rules (in this movie) of how the dead act. (They don't know they are dead, they don't see each other, etc). In fact, on second viewing, the movie and how people behave (like Willis' wife) actually makes MORE sense: A hallmark of a good twist.
So think of your hiding of Olivia's true identity as a twist, and write it like one. Not only is it okay: Audiences love a good twist.