Phoebe Caulfield prefers to sleep in her older brother D.B.'s room when he is away because his room is the biggest in the house. She likes to sit at his enormous desk to do her homework. Holden tell us that Phoebe doesn't like her own room because its not big enough.
"She doesn't like her own room because its too little, she says. She says she likes to spread out. That kills me. What's old Phoebe got to spread out? Nothing." (Salinger)
I think that Phoebe likes to sleep in her brother's room because she misses him. Consider, Phoebe had three brothers, Allie, Holden and D.B. Allie died, Holden is away at school and D.B. is away in Hollywood writing scripts for movies. She is left in the apartment with her parents, and she is lonely for her two remaining brothers, so she sleeps in her oldest brother's room because she gets to sleep in his big bed and use his fancy desk.
In the book it says that she dislikes her room because she thinks it is too little. This might represent the fact that Phoebe is free thinking and loves to have more than enough space for herself.