This creepy Guillermo Del Toro-produced horror flick (his hallmarks are all around the smudgy edges) demonstrates convincingly that step-parenting is a real bitch. Especially when a deranged she-demon from beyond the grave has laid all the groundwork. The step-mom is Annabelle (Jessica Chastain, miles away from The Help or Zero Dark Thirty) who couldn't be less maternal, as she swigs beer straight from the bottle and jams out with her punk rock pals. Alas, motherhood is thrust upon her when her artist boyfriend's orphaned nieces, long given up for dead, are discovered living alone in the woods like grubby changelings. Annabelle does her best to put up with — and eventually care for — the two girls, but it becomes increasingly obvious that someone . . . or something. . . has gotten there first. First-time director Andrés Muschietti makes the scares in Mama too obvious to be effective, but the quiet in-between times suggest genuine horror, as the hapless Annabelle tries to undo some seriously fucked-up pre-parenting.