We Have Always Lived in the Castle - Penguin Modern Classics by Shirley Jackson

Eighteen-year-old Merricat may, or may not be, a mass murderer.

Six years ago everyone in the Blackwood family was poisoned by sugar laced with arsenic – everyone, that is, apart from Merricat and her elder sister Constance. They live in peaceful, ordered isolation, away from prying eyes in the nearby village, until one day boorish cousin Charles arrives with designs on their father’s fortune. Whether by practical or magical means, Merricat will do whatever is necessary to protect their home.