Little Gods by Jenny Ackland

Olive Lovelock has just turned twelve. She is smart, fanciful and brave and on the cusp of something darker than the small world she has known her entire life. When she learns that she once had a baby sister who died — a child unacknowledged by her close but challenging family — Olive becomes convinced it was murder. Her obsession with the mystery and relentless quest to find out what happened have seismic repercussions for the rest of her family and their community. As everything starts to change it is Olive herself who has the most to lose as the secrets she unearths multiply and take on complicated lives of their own.