The wicked ones / Robin Benway.

'Blood is blood ... and one way or another, we all bleed.' Drizella and Anastasia only know one thing for certain: they will never end up like their mother, Lady Tremaine. When their father left them as young girls, he took what was left of their family's fortune and their mother's dignity with him. A few years and one deceased stepfather later, the only version of Lady Tremaine that Drizella and Anastasia know is a bitter and cruel head of house. Anastasia and Drizella have promised themselves-and each other-that they'll be different. They'll find love, see the world and never let their hearts go cold. But never underestimate the power of a mother whose greatest talents lie in manipulation, and the sisters may learn that even the cruellest of hearts can spill blood.

13+ years old.