Bruno's grandparents are shown as being loving parents of Bruno's Father, and doting on their grandchildren, but also being not entirely happy with the choices made by Bruno's Father. They are self-made, with Grandfather being a restaurant owner and Grandmother being a singer; they met before World War I and they remember the dangers of fascism. However, Grandfather seems to be more accepting of Bruno's Father as a member of the Nazi Party; Grandmother refuses to view this choice as acceptable and disrupts a Christmas party because of it:

"Standing there in your uniform," she continued, "as if it makes you something special. Not even caring what it means really. What it stands for."

"Nathalie, we discussed this in advance," said Grandfather, although everyone knew that when Grandmother had something to say she always found a way to say it, no matter how unpopular it might prove to be.

"You discussed it, Matthias," said Grandmother. "I was merely the blank wall to whom you addressed your words. As usual."
(Boyne, The Boy in the Striped Pyjamas, Google Books)

Despite their years of loving marriage, they are driven somewhat apart by the rise of the Nazi Party, and Bruno's Father is unable to reconcile with Grandmother before her death. The gap between Grandmother and Grandfather is important in understanding Bruno's Father and his reactions to the Nazis. Grandfather views the Nazis as a patriotic renewal of Germany's world stature; he sees Bruno's Father as a patriot and is proud of both his status and the work he will do. Grandmother is ashamed of Bruno's Father, as he is working towards the genocide of an entire culture of people; she sees past the propaganda to the actual events and results, and hates both the uniform and the innate brutality it represents.