The Demon's Lexicon Trilogy by Sarah Rees Brennan

Nick and his brother, Alan, have spent their lives on the run from magic. Their father was murdered, and their mother was driven mad by magicians and the demons who give them power. The magicians are hunting the Ryves family for a charm that Nick's mother stole -- a charm that keeps her alive -- and they want it badly enough to kill again. Danger draws even closer when a brother and sister come to the Ryves family for help. The boy wears a demon's mark, a sign of death that almost nothing can erase...and when Alan also gets marked by a demon, Nick is des-perate to save him. The only way to do that is to kill one of the magicians they have been hiding from for so long.
Ensnared in a deadly game of cat and mouse, Nick starts to suspect that his brother is telling him lie after lie about their past. As the magicians' Circle closes in on their family, Nick uncovers the secret that could destroy them all.
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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. 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?

The Demon's Surrender is the thrilling and suspenseful conclusion to the Demon's Lexicon trilogy.