Final answer:
Cinderella was sent to sleep in the cinder-covered kitchen, earning her the nickname from her sisters due to always being dirty with cinders. She still offered to help her stepsisters prepare for the royal ball.
Step-by-step explanation:
Cinderella's stepmother sent her to sleep in the cinder-covered kitchen, which is why her sisters mockingly gave her the nickname Cinderella. They called her this because she would always be dirty and covered in cinders from sleeping by the fireplace. Despite her sisters' cruel treatment, Cinderella was kind-hearted and offered to help her stepsisters get ready for the royal ball. She helped them with their hair and their dresses, hoping they would find happiness at the ball.