On Christmas Day, before Father became Commandant of the concentration camp, he wore his new uniform for the first time. The whole family applauded, and Bruno noticed that the soldiers seemed to show his father more respect when he wore it. Grandfather was very proud of him and told him he was patriotic, but Grandmother told him that he was "Dressing up like a puppet on a string" (pg 90) Grandfather comes to his defense telling him how proud he is of his son who is helping to reclaim the country's pride. But Grandmother says, "...to see you in that uniform makes me want to tear the eyes from my head." She stormed out of the house, and Bruno didn't see much of his Grandmother after that, and she never forgave her son. When she died, Bruno's mother said it was very hard on his father because he never made amends with his mother (pg 177)
when they're at Father's "promotion" party, the family, which includes: Father, Mother, Bruno, Gretel, Grandmother, and Grandfather; they start having a conversation about Father's "promotion". Grandfather talks about how proud he is of his son. Grandmother on the other hand is not so thrilled. She says that she disgraces it, and thinks it's inhumane.
Now your next question would be how does this relate to the resolution of the story?
It's ironic because in the end of the story Father sees how Grandmother is right. When Bruno goes into the gas chamber, he dies and Father realizes how killing innocent people, such as 8-year-old boys, is wrong.