Heartland Sunset by D. K. Graham
Jaina Wilson is a teenaged girl who has always thought her very best friend was the little girl in the mirror. As she grew older, she realized that it was just a figment of her imagination, but she does miss "Nellie", the girl in the mirror. She had always been able to go to her when she had something she needed to share with another being. Because of her parents' murders, she lives with her uncle and aunt, and moves with them to a small town in Kansas at the beginning of the story.
As the story unfolds, through a huge misunderstanding, Jaina unexpectedly meets another girl who looks just like her. When the two girls recover from their shock, they compare life stories and discover that they are, in fact, twins who were separated at three years of age, after their parents were killed in an explosion. Jaina was never told about her twin, who she now realizes was "Nelly, the girl in the mirror". Jainell was told that her twin, Jaina, also perished in the explosion.
The story follows the girls as they uncover the truth that their parents were actually murdered by the man who calls himself Jainell's father. Cammie, who Jainell has always thought of as her step-mother, finally admits that she had known the real story for quite some time, but that she feared her husband's abuse if she told anyone about it. After his death, there was nothing to stop her from letting everyone know the truth.
The story involves loss on several levels, including human deaths and loss of homes and possessions. It follows the family as they get to know each other through many tragedies. The story ends on an up-note with Jaina coming to terms with the tragic loss of her cousin and best friend, Lonnie.