Blood Sinister by Celia Rees
Ellen's just sixteen, but she's dying-and nobody knows why. So when she's sent to her gran's house she's relieved to find a distraction in the form of her great-grandmother's diaries, written when she was Ellen's age. As Ellen reads, she becomes increasingly embroiled in the fascinating story of her life. Who is the enigmatic and seductive Count her great-grandmother was supposed to marry? And why is it that Ellen and her great-grandmother resemble each other so much that they could be twins? But Ellen's condition is worsening, and it's becoming more and more difficult for her to distinguish her own life from the nightmarish events recorded in the diaries. Is the past destroying her, or do the diaries hold the key to Ellen's survival...?