What are u asking specifically..
Since its a sibling, and siblings tend to be in a person's close circle of people, informal speech is used.
As Holden shares his experiences, it becomes evident that he is talking from a mental facility where he is being psychoanalyzed. He is mentally insane, or at least considered so. This can be seen by his frequent use of the word 'madman" <span>when he criticizes himself: "I was smoking like a madman"</span>