The Wickeds by Gayle Forman

Two women, the wicked witch who fed the poisoned apple to Snow White and the wicked step-mother of Cinderella join forces to ruin the wedding of one of their grandchildren.  They join forces with the dwarves, who didn’t like Snow White barging in on their operation.  At the wedding they find that their granddaughters are spoiled brats and switch sides to make sure everything turns out all right in the end.