Book about a girl living with her bedridden mother and grandmother, and how a pregnant artist moves to their isolated island

Hello everyone, I read this book a year ago, if I remember correctly. It's about an isolated island where people inbreed and as a result, there's a lot of birth defects in the people on the island. The story is about a young girl whose mother is bedridden and she lives with her grandmother. Her father visits them every weekend from the mainland. The girl is twisted in the head and tries to hurt her mother. One day a pregnant artist moves to town and the girl starts obsessing over her and tries to get close to her. The artist pities the girl and starts to take care of her. The girl eventually ends up killing her mother by hanging her and leaving the island. Please help me find the name of the book. I'll be really grateful if you do.