Beyond the Reflection's Edge by Bryan Davis.
(Echoes from the Edge)
Sixteen-year-old Nathan Shepherd has a great life traveling where the careers of his father, an investigator, and mother, a renowned violinist, take him ... until his parents are found murdered. Left with only a mirror and notes from his father's last case, Nathan goes into hiding at the remote country home of Tony, his father's college buddy, and Tony's teenage daughter, Kelly. The mysterious mirror must be a clue to what happened to his parents, and when images appear in it---people and things that don't exist---Nathan and Kelly painstakingly gather evidence. But the killers want the mirror too, and danger threatens the teens at every turn. As it becomes evident that Nathan's father had stumbled upon dark forces at work in the world, several questions arise. Could it be that the mirror is a portal to a parallel world? Could this technology be used for evil purposes? And could his parents still be alive?