Dangerous Remedy by Kat Dunn

Blurb:

Camille, a revolutionary's daughter, leads a band of outcasts – a runaway girl, a deserter, an aristocrat in hiding. As the Battalion des Mortes they cheat death, saving those about to meet a bloody end at the blade of Madame La Guillotine. But their latest rescue is not what she seems. The girl's no aristocrat, but her dark and disturbing powers means both the Royalists and the Revolutionaries want her. But who and what is she?

In these dangerous days, no one can be trusted, everyone is to be feared. As Camille learns the truth, she's forced to choose between loyalty to those she loves and the future.