The most evident answer is that the relationship between all four characters is that they are a family, the central family, in Boyne's novel. They are all linked by blood and the relationship they share. As a family of four, they are the center of the narrative. On a more symbolic, the German family might be seen to represent the different aspects of German society during the Holocaust. Father is a believer in Nazism, representing the significant part of German society that embraced Hitler's ideology and what it meant. Gretel is one who becomes infatuated with the power that accompanies Nazism, not fully aware of its implications. The mother of the family struggles between the support of her husband and the need to shelter her family from what she knows is wrong. Bruno could come to represent the portion of German society that stood up to the Nazis. While not significant in mass numbers, there were Germans, good Germans, who recognized what Hitler was doing as wrong and stood up, paying the ultimate sacrifice, because of it. I think that this might be where the relationship between the family members can come to represent more than the central characters in the Boyne novel.