The Fairy-Tale Detectives by Michael Buckley

In which two orphaned sisters, of the family Grimm, after being shunted from one foster family to another are finally claimed by a woman who says she is their grandmother.  The problem, as far as the older sister is concerned, is that their father told them their grandmother is dead.  The grandmother’s driver is a gaunt old man and her car seems even older.  In the town, which also looks very old, the girls get a lot of odd looks.  The mayor is Mr. Charming, there are other people with names of characters from fairy tales.  In their grandmother’s house, there is a room they can’t go into.  The older sister is waiting for the chance for her and her sister to escape yet another crazy foster home.  But then a giant steps on a farmhouse and the story really gets wild from there.  All the while, the older sister refuses to believe in fairy tales, until the giant steals their care with their grandmother and her driver right in front of them.  The grandmother’s keys come flying out and onto the ground.  They have no choice but to go back to their grandmother’s house and see what is in the forbidden room.  And it’s the magic mirror, hiding all kinds of magical artifacts in a warehouse behind the mirror.  Jack, from the beanstalk tale is the only one in town who knows how to kill a giant, and he’s in jail.  The girls have to rescue their grandmother and figure out who is causing all the trouble.