Inconsistencies
While a prequel, Amityville II contradicts the opening of the 1979 film which shows the family massacre, and like the actual event they are all sleeping. The bodies are also removed from the house in the morning, but in the first film, it's in the middle of the night.
Some of the family drama in the film did happen to the Defeos, but are exaggerated. The story introduces speculative and controversial themes, including an incestuous relationship between Sonny Montelli and his teenaged sister, who are based loosely on Ronald DeFeo, Jr. and his sister, Dawn and a rumored incestous relationship.
Though supposedly set one year before the first Amityville Horror film, Amityville II is full of elements that are clearly from the 1980s, such as a miniature "Walkman" radio/headphone set and the presence of 1982 cars, televisions, etc.
Read more about this topic: Amityville II: The Possession