context: On August 6, 1956 the Grandmother dies of a major upper-class family in Cali. The descendants are called to hear the will of the grandmother, two of the heirs are Andres Alfonso and his half sister Margaret. At dawn on August 7 of that year the infamous explosion occurs several truckloads of dynamite that were to be transported to BogotÃ¡. No light, no water and with the roof partially fallen's family as well as others head into the field. Andres and Margaret are asked to go to the farm-out "La Emma" to go for supplies and give the news to the great-uncle Enrique of the death of his sister and his share of the inheritance. Both come to be greeted by the caretaker of the farm and later directed where Enrique is known as the black sheep of the family for being different from them. Both being dirty clothes are changed while Henry tells of its history and let them see both photos and memories. From there it develops a Gothic story of love between Andres and Margaret that slowly develop a strong attraction resulting in an incestuous relationship in which little by little Andres and Margaret are the ghosts of their ancestors who are taking possession of both. The lovers disappear and appear in both the farm and in other murdering and spreading terror in the region. A historical realism magic to resurrect become symbols of power - the matron pride, the bloodthirsty general, ethereal nuns and the doctor vampire.
  Both become cannibalistic and vampiric creatures who spread terror to be confused with indigenous myths as Madremonte. By taking a victim's son of a peasant family in the service, the brothers are killed and buried by this. In addition to the police station, the family seeks to Andres and Margaret, however no reason for them except the main suspect was the peasants who worked in the family. Andre's dad calls look so touched arrest all but the commander calls on those present to leave. A farmer discovers previously terrified without knowing the site where two teenagers were buried and left it shocked when lovers are raised and out of his grave to keep on sowing terror.
question: What relation are Andres and Margaret?
answer: Gothic story of love