In The Catcher in the Rye, it's fair to say that Holden's relationship with Phoebe is the only close one that he has. Whether it's his parents, teachers, or the adults that he happens to meet on a daily basis, Holden is chronically incapable of establishing close connections with anyone else.

To a large extent, this is because Phoebe isn't a part of the adult world that Holden positively loathes and that he's never really understood. For Holden, this is a sham world, a world full of phonies. But Phoebe's not like that; she's still young and innocent, the kind of child that Holden fantasizes about preventing falling off a cliff. For good measure, she seems to understand Holden in a way that no one else ever could.

One could also say that Holden's closeness to Phoebe derives to a considerable extent from the painful loss of his late brother, Allie, who died tragically young of leukemia. Having lost one sibling, Holden doesn't really want to lose another. And so he remains fiercely protective of Phoebe, even though he eventually realizes, as he watches her on the carousel at the end of the story, that he has to let go and let her live her own life.

Holden Caulfield and his younger sister, Phoebe, share a close relationship even though he has been away at boarding school. The novel does not reveal what their relationship was like before the death of their brother Allie. Since Allie passed away from cancer, Holden has come to feel extremely protective of Phoebe. His concern for her has prompted his broader desire to protect all children—the sensation that he describes as being the “catcher” who will save children in danger of falling off a cliff.

When he returns home after being expelled, Phoebe is the confidante he seeks out. Although Holden has little to say about their parents, it is clear that he does not trust them the way he trusts her. Phoebe is one of the few people with whom Holden can truly be himself. Because he does not consider her to be one of the “phonies” he detests, he doesn't try to pretend he is more grown up, nor does he speak sarcastically to her.

The extent of their trust and closeness is shown by Phoebe’s support—albeit reluctant at first—of Holden’s plan to run away. Her greater practicality is shown by her having more money saved than him. After she gives him the money, however, she decides that she will miss him too much and should leave with him. Phoebe’s action is instrumental in turning things around for Holden, as he realizes how impractical his idea was and that by staying home, he has a better chance of “catching” both his sister and himself.

Chapters 21 through 25 make mention of Holden Caulfield's sister Phoebe, "old Poebe" as he affectionately calls her. It is clear from Holden's descriptions and remarks about his little sister, that he loves her. 

• Phoebe is an ingenuous child, who readily and honestly displays her feelings. When Holden sneaks into the apartment and wakes Phoebe, she hugs him immediately.

She always listens when you tell her something. And the funny part of it is she knows, half the time, what the hell you're talking about. She really does.

• Phoebe is honest and candid with Holden, not "phony," a characteristic that he loathes in people. When Holden tells Phoebe that he did not like anything that was happening at the school he has run away from, Phoebe remarks, "You don't like anything that's happening." And, she forces Holden to name something he does like, challenging him.
• Phoebe does not judge Holden; instead, she just listens. Even if she is not certain of the meaning of all that Holden says, she listens. While she scolds her brother about leaving school, saying "Dad's going to kill you," Phoebe still does not pass judgment upon him.
• After Holden is with Phoebe for a while, they dance and Phoebe is able to follow him well; this gives Holden much pleasure and he is proud of how well Phoebe can learn dance steps.

In between numbers, she is funny as hell. She stays right in position....

• Holden is always relaxed with Phoebe; in the stores, "she enjoys horsing around and looking at the people." One time in a shoe store, Phoebe tried on all kinds of storm shoes, frustrating the salesman; finally, she bought another type of shoe. Her antics amuse Holden.
• Most of all, Phoebe loves Holden, kissing him and wanting to be with him, even willing to run off with him. In the final chapter, Holden watches his little sister on a carousel, feeling happy because "she looked so...nice."

Holden's and Phoebe's relationship is a close, loving one, but, ironically, Phoebe is often more mature than Holden, who is years older than she. Certainly, Phoebe is Holden's emotional anchor; for, with his sister there is no need for pretense or embarrassment.