The magic in books has to follow rules. That's what makes these books like games: The characters have to figure out how to win the quest or contest (or defeat the evil forces) by using magic correctly. But sometimes the rules underlying the magic don't quite make sense.
That's the case with Michael Buckley's Sisters Grimm series. The basic magical rule in this series is that fairytale characters, called Everafters, are trapped inside a town in upstate New York, Ferryport Landing, for the safety of the world. When they get to a border, it's like hitting a glass wall. They cannot leave this town as long as members of the Grimm family, descended from the famous Brothers Grimm, also remain in town. If the Grimms are gotten rid of, all the Everafters can go free.
It's not hard to find the flaw in this setup— one family of humans against hundreds (or thousands) of magical people and creatures? And who can blame the Everafters for resenting the Grimms? Of course they gang up against them. The Grimms gather some allies, but it's hard to figure out how. These allies seem to side with the Grimms against their own best interests.
To add to this mess, many Everafters don't even live in Ferryport Landing. They live in the great wide world, especially New York City, but they become trapped in the F.L., kind of like a roach motel, if they come within its borders.
As a result, although this series has some fun characters and humor and excitement, the magic doesn't work that well for me. The magic game is untenable.