We Have Always Lived in the Castle 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 fathers fortune. Whether by practical or magical means, Merricat will do whatever is necessary to protect their home.