Final answer:
Gregor's family turns against him in Part III of 'The Metamorphosis' due to the overwhelming burden his transformation causes, leading to resentment and a sense of hopelessness.
Step-by-step explanation:
In The Metamorphosis, Gregor's sister and father turn against him in Part III primarily because of the burden his transformed state imposes on the family. Initially, there is a mix of fear, pity, and familial duty that keeps them caring for Gregor. However, as time progresses, the strain of Gregor's needs and the alienation caused by his grotesque form increasingly wear on the family.
The family's financial struggles, the shame of Gregor's condition, and the loss of hope for a return to normalcy culminate in feelings of resentment and despair. This deteriorating situation leads to a shift in the family's attitude from caring to rejection as Gregor's existence becomes an unbearable weight on their lives.