Albert Barnes on Acts 23:16

Paul's sister's son. This is all we know of the family of Paul. Nor do we know for what purpose he was at Jerusalem. It is possible that Paul might have a sister residing there; though, as Paul had been sent there formerly for his education, it seems more probable that this young man was sent there for the same purpose.

Entered into the castle. Paul had the privileges of a Roman citizen; and as no well-founded charge had been laid against him, it is probable that he was not very closely confined, and that his friends might have free access to him.

(a) "he went" 2Sam 17:17