Twins, Roslyn Chapel, descendants of Jesus of Nazareth
Adult mystery about the secret descendants of Jesus of Nazareth. A girl and her grandfather are the main characters. At the end of the book they come to Roslyn Chapel where they go into a hidden underground chamber and where they meet the girl’s grandmother and twin brother, whom she didn’t know existed. It’s not The Da Vinci Code, I must have read it 10-15 years earlier.