In Franz Kafka’s The Metamorphosis, Gregor’s sister, Grete, initially takes care of him and even protects him in his bug form. However, by the end of the story, she is decidedly against him and even encourages her parents to get rid of him. What theme of the story is most clearly hinted at here?

Grete’s treatment of Gregor is the most pertinent example of rejection, a key theme of Kafka’s The Metamorphosis. While the rest of the family immediately distances themselves from Gregor, his sister gradually rejects Gregor in his new form.
Grete is the first to speak to Gregor after he has been transformed. However, this communication is angry and serves to further isolate him from the family. His mother has seen him and fainted as a result. Kafka implies that Grete wants to inflict the same feeling on Gregor: “Grete now felt tempted to want to make Gregor’s situation even more terrifying.”
Midway through the story, Kafka writes “surely no one except Grete would ever trust themselves to enter a room in which Gregor ruled the empty walls all by himself.” The theme of rejection operates as a two-way street in the story, as Gregor’s new form also isolates Grete from her brother. The family also encourages this rejection within themselves, as Grete and her father prevent the mother from going to see Gregor.
After Gregor’s death, the family takes a train ride through the countryside. As they discuss their dreams and future plans, Kafka closes the story with a description of Grete having moved on to a better life:

And it was something of a confirmation of their new dreams and good intentions when at the end of their journey the daughter first lifted herself up and stretched her young body.

This tells the reader that Grete has experienced a transformation of her own, as she has moved from Gregor’s sole caretaker to an ambitious, independent young woman.