Paul’s sister’s son - This is all that 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 himself 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.