The Third Twin By Ken Follett

Using a restricted FBI database, genetic researcher Jeanie Ferrami has located identical twins born to different mothers. Frightened by her bizarre discovery, she is determined to discover the truth at any cost—until she finds herself at the center of a scandal that could ruin her career. To fight the charges, Jeannie plunges into a maze of hidden evidence. With growing horror, she uncovers a cynical, far-reaching conspiracy involving disturbing genetic experiments and some of the most powerful men in America—men who will kill to keep their secrets concealed.