Girls in the Garden by Lisa Jewell

Imagine that you live on a picturesque communal garden square, an oasis in urban London where your children run free, in and out of other people's houses. You've known your neighbors for years and you trust them. Implicitly. You think your children are safe. But are they really? On a midsummer night, as a festive neighborhood party is taking place, preteen Pip discovers her 13-year-old sister Grace lying unconscious and bloody in a hidden corner of a lush rose garden. What really happened to her? And who is responsible?