The Fairy-Tale Detectives by Michael Buckley

In this first installment of The Sisters Grimm series, sisters Sabrina and Daphne Grimm have had far from fairy-tale lives. They don't expect it to be much different when they get shipped off to live with yet another stranger...although this one, claiming to be a relative the girls are sure they don't have, just might prove to be the strangest. Their granny turns out to be the least of their worries, however, when they find it that they are not only related to the famous Brothers Grimm, but that also all the fairy tales they created weren't actually fake - and the dangers posed in those tales are still quite real.