Book about a babysitter + chamomile + gardening and grave with lillies

Guys Im looking for a similar book, Its about a girl who goes to baby sit an "ill girl" and somthing I remember is that The mother of the girl always drank chamomile and LOVED gardening but she was kinda weird, and that they had a son but he didn't visit often and their daughter is weird, she drawins with black ink on the walls and stuff, and I remember when the girl found out the secret of the family that the mom actually gives her daughter some sort of medicine that wont allow her to talk or control her body well she ran away but the mother caught the babysitter and tried to drown her in their backyard lake PLEASE help! It onvolves a garve with lilies and grandma