Into the Woods by Lyn Gardner is a solid GU story with the extra bonus of including a lot of fairytale references – in varying degrees of subtlety – which the reader can enjoy searching out.
Storm lives with her extremely oblivious parents (a GU hallmark) and older sister. One day she happens to see a town meeting where they hire a sinister man, Dr. DeWilde, and his ravenous wolves to help with the rat infestation. Soon after, her mother dies in childbirth, after bringing into the world her new sister Any, and bequeathing Storm a magical tin pipe. Their father soon runs off, unable to cope, and the sisters are left to fend for themselves.
When Any is taken by DeWilde and his minions, Storm leads Aurora on a chase to rescue her, helped along the way by an old ogress who is kinder than she seems. Storm eventually goes underground via an old mine shaft into the heart of a mountain. Then she manages to lose her older sister too and must rescue them both. But she doesn’t stop there, and has a final confrontation with her adversary where she exposes him for what he truly is, regains the pipe he had stolen, and leaves him to his painful end.
I just found out there is a sequel that also appears to be a potential GU story in its own right, so I may add that too at a later date.