Gretel is Bruno's older and, as far as she's concerned, wiser sister. She and Bruno don't talk much, even though there's nobody else around to hang out with, and for the most part, Gretel serves as a source of information for Bruno. Unfortunately, though, this information is oftentimes muddled or just plain wrong. Despite seeing herself as a fount of wisdom, Gretel's still just a kid, after all. Insofar as Bruno is a young German and Gretel is in a sort of leadership position, then, she represents how misguided German leadership is at this time.
Despite not wanting a whole lot to do with her little brother, after they move, Gretel is sort of nice to Bruno when they arrive:
Gretel looked at her little brother and found herself agreeing with him for once. "I know what you mean […] It's not very nice, is it?" (3.115)
She commiserates with her brother about their plight in being forced away from their friends, but when Bruno shows her the children he can see from his window in Chapter 4, Gretel fails to understand what they're seeing.
At first she suggests Auschwitz is some sort of farm, but when Bruno points out this can't be (no animals or crops, after all), she concedes he's probably right. Instead of considering a darker explanation, though, she readily agrees with her brother's assessment that perhaps they're all rehearsing for some sort of performance as soldiers scream at children in the camp. And then she leaves to go play with her dolls. Gretel's had enough sibling bonding for one day, thank you very much.
Gretel's interests mature from dolls to geography, and she starts learning about the war. Frustratingly, though—for us and forBruno—she keeps much of what she learns to herself. Classic older sister move, right? Instead of using her newfound knowledge to enlighten Bruno, Gretel uses it to elevate her own standing, to separate herself from her pesky kid brother.
For instance, she explains to Bruno that, "The fence isn't there to stop us from going over there. It's to stop [the Jews] from coming over here" (12.1088). Sounds like she's off to a good start, right? She's certainly no longer super confused about what's happening next door. And yet, when Bruno asks her why people don't like Jewish folks, she simply says: "Because they're Jews (12.1109). Oy vey—thanks for nothing, sis.
In Gretel's defense, as she's learning about the war and the concentration camps, she's also coming up against some really hard truths about her father and his work. It may be that she's just not quite ready to dig deep into the reasoning behind her father's work—to do so presents her father in a pretty unfavorable light. And while she's growing up, she may feel resistant to really confronting who and how her father is in the world.