Here's a bit about the novella from Goodreads:
Twin sisters Jack and Jill were seventeen when they found their way home and were packed off to Eleanor West’s Home for Wayward Children.
This is the story of what happened first…
Jacqueline was her mother’s perfect daughter—polite and quiet, always dressed as a princess. If her mother was sometimes a little strict, it’s because crafting the perfect daughter takes discipline.
Jillian was her father’s perfect daughter—adventurous, thrill-seeking, and a bit of a tom-boy. He really would have preferred a son, but you work with what you've got.
They were five when they learned that grown-ups can’t be trusted.
They were twelve when they walked down the impossible staircase and discovered that the pretense of love can never be enough to prepare you a life filled with magic in a land filled with mad scientists and death and choices.
We have quite a while to wait for this one since it's not due out until June, but that's plenty of time to go ahead and read its predecessor in the Wayward Children series if you haven't yet. Go ahead, go buy it. You won't regret it. McGuire is brilliantly creative and Every Heart a Doorway proves that in spades!