Vanished

Clara receives a letter one afternoon telling her that all her family had been murdered that morning and that a book that had been passed down in her family for centuries holds all of the answers to her questions ... but the book had been lost for decades.
How did the person that sent the letter know about all of this? Should she really believe what the letter says? Her family couldn't be dead .... could they?

Even if they were ... why was she the only one left alive?