context: On a stormy night in Wales, five people, Philip and Margaret Waverton, their friend Penderel, Sir William Porterhouse and his lady friend, chorus girl Gladys Perkins, whose stage name is DuCane, seek refuge in a gloomy house off the road. The denizens of the house include Horace Femm, an hysteric, his sister Rebecca, a religious fanatic, and Morgan, their scarred, brutish butler, who is a mute. At dinner, Horace confides that sometime in the past, their sister Rachel died in a mysterious fashion. As the evening progresses, the Wavertons discover the Femms' 102-year-old father in an upstairs room. Transformed by drink, Morgan pursues attractive Margaret up the stairs, where he craftily releases another brother, pyromaniac Saul, from his locked room at the top. Penderal and Gladys have fallen in love at first sight. They break the news to Sir William who, because he is still in love with his dead wife, is not very upset. Soon after, Penderal encounters the liberated Saul in one of the dark rooms. At first Saul seems to be the only sane inhabitant of the house, but he proves that he is as crazy as the rest when he tries to kill Penderal. Both men are wounded in the fight that follows. Morgan, having sobered up a little, carries the wounded Saul back to his room. After Gladys treats Penderal, dawn finally breaks and the storm is over. The five guests leave the house behind as quickly as possible and Horace cheerfully bids them goodbye, as if the events of the night had never happened.
question: How are Horace and Rebecca related?
answer: Brother and sister