All That's Bright and Gone by Eliza Nellums

Aoife is a precocious six-year-old caught in an adult world she doesn't understand but with the help of her imaginary friend Teddy, her kind uncle and others she is trying to make sense of it. Her mother has gone away because she has been sad ever since Aoife's older brother left. The way Aoife sees the world is that if she can solve her brother's murder, her mom will be happy and she will come home.
Narrated by a six-year-old who is every bit her age and as wise as a sixty-year-old this is a touching story of a family learning to deal with what life has thrown at them and trying to stay together when everything seems to be pulling them apart. The characters feel real and the situation they find themselves in is all too real for many families.