The answer you have been already given is a good answer to the question you asked; however, you have your characters mixed up as Bruno is Gretel's brother while Schmuel is the Jewish boy behind the fence who wears the striped pajamas. I believe that the question you want answered is more about how Bruno feels about his sister Gretel and her flirtation or romance with a young Nazi soldier. Bruno meets Adolf Hitler with Eva Braun at his house at a dinner. Gretel is quite taken with Hitler and tries to get his attention. Bruno, on the other hand, does not like Hitler and is disgusted with his sister's behavior. Bruno also is disappointed with his sister's flirtation with the young Nazi soldier who is too much like Hitler. He calls his sister "the Hopeless Case" because their views of people and situations are so different and his sister so flirtatious. Bruno is disgusted but never really discusses his sister's marriage.
The Boy in the Striped Pajamas is set during World War II, and the bulk of the novel is set in a place the young protagonist, Bruno, calls "Out-With." This is really the infamous Auschwitz concentration camp, and Bruno's father is now the commandant in charge of the camp, though Bruno does not really figure that out.
Bruno is nine years old and desperately wishes for a friend. The closest thing he has to a friend is his older sister, Gretel. He calls her a "Hopeless Case," and the two of them do not have any real relationship for most of the novel. Gretel is flirtatious and seems to understand a little more about where they are and what her father does for a living.
One day Bruno begins exploring, one of his favorite things to do. In the distance he sees a kind of a blob.
And shortly after that the blob became a figure. And then, as Bruno got even closer, he saw that the thing was neither a dot nor a speck nor a blob nor a figure, but a person.
What Bruno discovers is a little boy, wearing what Bruno thinks are striped pajamas, sitting along the fence of the camp. We learn that the boy's name is Shmuel and he is exactly the same age as Bruno.
Over the course of the novel, Bruno and Shmuel talk about many things, but they never talk about Gretel and who she should marry.