I think one of the most significant items of information we can glean from Chapter Twenty One comes towards the end, when Phoebe realises with typical astuteness why Holden is here two days early, when he was supposed to have returned on Wednesday. It is obvious that Phoebe is an intelligent girl, and Holden's attempts to deceive her end in failure as she realises the truth. Note what she says when she realises that her brother has, yet again, been kicked out of his school:
"You did. You did," she said. Then she smacked me again with her fist. If you don't think that hurts, you're crazy. "Daddy'll kill you!"
It is clear from this exchange and Phoebe's disappointment with her brother that their father is going to be very angry when he discovers that his son has once more been expelled from a school and that he is likely to be very angry. This shows us the impact of Holden's actions om his poor family, who obviously care for him and want what is best for him, but clearly seem puzzled and perplexed as to how best to help him.