The way that Bruno's sister, Gretel, embraces Nazism is one way she changes throughout the story.  

When we are first introduced to Gretel, she is excited about dolls and seems to embody the tendencies of a typical child.  In her role as the "hopeless case," the relationship she shares with Bruno fits the profile of the occasional antagonism between brother and sister.  

She changes with the family's move to Auschwitz.  She starts to fervently embrace Nazi ideology.  Hitler Youth posters fill the walls of her room as she deems her doll collection as infantile.  She views Nazism as a way to gain popularity and social acceptance.  She becomes more concerned with appearances, as seen in her idolizing Lieutenant Kotler.   Gretel's approach to the world changes because she sees Nazism as a path to social acceptance and popularity.

The changes that Gretel experiences are representative of how seductive Nazism was to the German public.  Her changes highlight how people can find social inclusion alluring.  Gretel's changes translates to an unwillingness to see the pain and suffering which were the results of Nazism.  

One more change in Gretel is seen when Bruno goes missing.  She is moved by the loss of her brother.  Despite her embrace of Nazism, she goes back with her mother to Berlin.  Perhaps, this change reminds the reader that no matter what, many could not fully avoid the cruelty and savagery intrinsic to Nazism. Gretel's departure from Auschwitz and the sadness she felt at Bruno's absence shows that, despite her changes, Nazism did not completely erode her sense of decency.