While we are not given a direct description of Gretel's appearance in the novel, here are some details about her character that may help to form an image of Bruno's older sister for you:
In John Boyne’s novel The Boy in the Striped Pajamas, in addition to being repeatedly referred to by her brother Bruno as a “Hopeless case,” and, at the beginning of Chapter 3 as “nothing but trouble,” Gretel is further described in that chapter as Bruno’s older sister by three years, as the following text from the book demonstrates:
“Gretel was three years older than Bruno and she had made it clear to him from as far back as he could remember that when it came to the ways of the world, particularly any events within that world that concerned the two of them, she was in charge.”
As Bruno is described in this chapter as being nine-years-old, that, obviously, makes Gretel 12-years-old. While Bruno clearly chafes at life under the domineering and occasionally taunting presence of his sister, he recognizes her as the more mature person with a better understanding of the ways of the world, at least until the family arrives at its new home, which happened to be the home of the newly-appointed commandant of the concentration camp where Jews were being held in preparation for their slaughter. As Boyne describes Gretel’s discovery, courtesy of Bruno, of their strange new surroundings, he provides another bit of information on this character:
“She stood still for a long time staring at them. She was twelve years old and was considered to be one of the brightest girls in her class, so she squeezed her lips together and narrowed her eyes and forced her brain to understand what she was looking at.”
Gretel will develop a schoolgirl crush on Lieutenant Kotler, the young aide to Bruno and Gretel’s father who displays arrogance towards the children and a deep hatred for the Jewish prisoners, despite the secret regarding his family that will be revealed in time. As a sign of her innocence and youth, Gretel possesses a large collection of dolls in her bedroom that Bruno was convinced monitored his movements. While Bruno discoveries the humanity in the Jewish prisoners by virtue of his blossoming friendship with Shmuel, Gretel remains an unrepentant junior Nazi, adorning her bedroom with Nazi paraphernalia and maps of Europe and newspaper articles she uses to follow the war:
"Gretel had decided that she didn't like dolls any more and had put them all into four large bags and thrown them away. In their place she had hung up maps of Europe that Father had given her, and everyday she put little pins into them and moved the pins around constantly after consulting the daily newspaper."
In the end of the novel, Gretel returns to Berlin with her mother, while father remains in command of "Out with," the sounds Bruno had heard that was actually "Auschwitz."
How is bruno presented in chapter. 1