I can't remember the name but the story of the murder is told in the film "Heavenly creatures" - see http://www.imdb.com/title/tt0110005/ Following the film, someone tracked down one of the two girls. She had changed her name, become an author and moved to Scotland.
Have found the following in the IMDB: Juliet Hulme was revealed to be mystery writer Anne Perry in 1994 during the making of the film but all attempts to find Parker failed. In 1997 she was traced to a rundown cottage on a farm near Strood, Kent, England, where she runs a children's riding school. Since assuming the name Hilary Nathan the spinster has become a devout Catholic and devoted her life to handicapped children.