The Problem Child, by Michael Buckley
Book Three of The Sisters Grimm actually is not a mystery. It's pretty much a straight adventure concerning the fallout of Sabrina's attempt to rescue her parents at the end of The Unusual Suspects. Most of the story, in fact, revolves around trying to escape from the creature memorialized in a poem by Lewis Carroll that begins 'Twas brillig, and the slithy toves. The reason I haven't named the creature in question? Because Buckley gets it wrong for 292 pages. Drove me crazy, and just about made me want to throw the book at the wall.
The creature is a Jabberwock. Jabberwocky is the title of the poem. Argh!
I guess I'm still a little irritated.
Maybe on my next reread I'll sit with a bottle of correction fluid next to me so I can cover up all of those "y"s. That'd probably make me feel better.
Things pick up, and become slightly less annoying, when they realize what they need to defeat the Jabberwock*. The ensuing confrontation ends up being kind of therapeutic for Sabrina, as well. Though she's still missing her lost "normality," as we see in the next book in the series, Once Upon a Crime.