The girl refers to her brother as "The Pain," due to him always being a nuisance to her, while the boy calls his sister "The Great One," a sarcastic nickname based on her thinking herself the better sibling. It is a humorous look at the mixture of emotions shared by young siblings.
The girl refers to her brother as "The Pain" and calls him annoying, while the boy nicknames his sister "The Great One," which stems from her belief that she is superior. Both scenarios illustrate the complex emotions experienced by siblings.