The Demon's Lexicon Series by Sarah Rees Brennan

Sixteen-year-old Nick and his brother, Alan, are always ready to run. Their father is dead, and their mother is crazy—she screams if Nick gets near her. She’s no help in protecting any of them from the deadly magicians who use demons to work their magic. The magicians want a charm that Nick’s mother stole—and they want it badly enough to kill. Alan is Nick’s partner in demon slaying and the only person he trusts in the world... moreSixteen-year-old Nick and his brother, Alan, are always ready to run. Their father is dead, and their mother is crazy—she screams if Nick gets near her. She’s no help in protecting any of them from the deadly magicians who use demons to work their magic. The magicians want a charm that Nick’s mother stole—and they want it badly enough to kill. Alan is Nick’s partner in demon slaying and the only person he trusts in the world. So things get very scary and very complicated when Nick begins to suspect that everything Alan has told him about their father, their mother, their past, and what they are doing is a complete lie. . . . less

Mae Crawford's always thought of herself as in control, but in the last few weeks her life has changed. Her younger brother, Jamie, suddenly has magical powers, and she's even more unsettled when she realizes that Gerald, the new leader of the Obsidian Circle, is trying to persuade Jamie to join the magicians. Even worse? Jamie hasn't told Mae a thing about any of it... moreMae Crawford's always thought of herself as in control, but in the last few weeks her life has changed. Her younger brother, Jamie, suddenly has magical powers, and she's even more unsettled when she realizes that Gerald, the new leader of the Obsidian Circle, is trying to persuade Jamie to join the magicians. Even worse? Jamie hasn't told Mae a thing about any of it. Mae turns to brothers Nick and Alan to help her rescue Jamie, but they are in danger from Gerald themselves because he wants to steal Nick's powers. Will Mae be able to find a way to save everyone she cares about from the power-hungry magician's carefully laid trap? [From Borders.com] less

The Goblin Market has always been the center of Sin’s world. But now the Market is at war with the magicians, and Sin’s place is in danger. Thrown out of the Market she loves, Sin is thrown together with brothers Nick and Alan—whom she’s always despised. Alan has been marked by a magician and is being tortured so that the magicians can get to Nick... moreThe Goblin Market has always been the center of Sin’s world. But now the Market is at war with the magicians, and Sin’s place is in danger. Thrown out of the Market she loves, Sin is thrown together with brothers Nick and Alan—whom she’s always despised. Alan has been marked by a magician and is being tortured so that the magicians can get to Nick. As Sin watches Alan struggle to protect the demon brother he loves, she begins to see him in a new light—but she and Mae are locked in a fierce rivalry over who will inherit the leadership of the Goblin Market, and a decisive battle with the Aventurine Circle is looming. Mae’s brother, Jamie, is holed up with the magicians, his loyalties in question. And Nick—well, who knows what a demon might do to save his brother? How far will Nick go to save Alan—and what will it cost them all? less