A Daughter's Choice by Francis June

Seventeen year old Katie is about to discover a devastating family secret... Katie is the apple of her mother's eye and is being groomed to take over the family business. But when Celia, her natural mother, re-enters her life, her world is turned completely upside down. Tormented by her divided loyalties, Katie is plagued by a question Celia refuses to answer - who is her real father? (Note: Originally published as Somebody Else's Girl)