Albert Barnes Commentary Acts 23:16

"But Paul`s sister`s son heard of their lying in wait, and he came and entered into the castle and told Paul." — Acts 23:16 (ASV)

Paul's sister's son. This is all we know of the family of Paul. Nor do we know for what purpose he was in Jerusalem. It is possible that Paul might have had a sister residing there; though, as Paul had been sent there previously 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 since 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.