Task: Question Answering
Input (English): Aubrey, a young Englishman, meets Lord Ruthven, a man of mysterious origins who has entered London society. Aubrey accompanies Ruthven to Rome, but leaves him after Ruthven seduces the daughter of a mutual acquaintance. Aubrey travels to Greece, where he becomes attracted to Ianthe, an innkeeper's daughter. Ianthe tells Aubrey about the legends of the vampire. Ruthven arrives at the scene and shortly thereafter Ianthe is killed by a vampire. Aubrey does not connect Ruthven with the murder and rejoins him in his travels. The pair is attacked by bandits and Ruthven is mortally wounded. Before he dies, Ruthven makes Aubrey swear an oath that he will not mention his death or anything else he knows about Ruthven for a year and a day. Looking back, Aubrey realizes that everyone whom Ruthven met ended up suffering. Aubrey returns to London and is amazed when Ruthven appears shortly thereafter, alive and well. Ruthven reminds Aubrey of his oath to keep his death a secret. Ruthven then begins to seduce Aubrey's sister while Aubrey, helpless to protect his sister, has a nervous breakdown. Ruthven and Aubrey's sister are engaged to marry on the day the oath ends. Just before he dies, Aubrey writes a letter to his sister revealing Ruthven's history, but it does not arrive in time. Ruthven marries Aubrey's sister. On the wedding night, she is discovered dead, drained of her blood - and Ruthven has vanished. 
Question: How does Aubrey attempt to tell his sister about Ruthven?
Output (English): in a letter