Lies Like Poison by Chelsea Pitcher

Poppy, Lily, and Belladonna would do anything to protect their best friend, Raven. So when they discovered he was suffering abuse at the hands of his stepmother, they came up with a lethal plan: petals of poppy, belladonna, and lily in her evening tea so she’d never be able to hurt Raven again. But someone got cold feet, the plot faded to a secret of the past, and the group fell apart. Three years later, on the eve of Raven’s seventeenth birthday, his stepmother turns up dead. But it’s only belladonna found in her tea, and it’s only Belladonna who’s carted off to jail… Read more.