A String in the Harp - Nancy Bond

When fifteen-year-old Jen Morgan flies to Wales to spend Christmas with her family, she's not expecting much from the holiday. A year after her mother's sudden death, her father seems preoccupied by the teaching job that has brought him and Jen's younger siblings to Wales for the year. Her brother, Peter, is alternately hostile and sullen, and her sister, Becky, misses Jen terribly.
Then Peter tells Jen he's found a strange artifact, a harp key that shows him pictures from the life of Taliesin, the great bard whose life in sixth-century Wales has been immortalized in legend. At first Jen doesn't believe him, but when the key's existence -- and its strange properties -- become known to the wider world, the Morgans must act together against a threat to the key...and to their family.