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 addresses her brother with the nickname "The Pain" because he often irritates her, whereas the boy calls his sister "The Great One," a term based on her self-perception as the better sibling. This depiction highlights the emotional interactions between young siblings in a humorous context.