Brother may have been a normal greeting amongst Jewish men. In this case however it is a recognition from Ananias that the Lord has revealed himself to Saul and that Saul is now a Christian and part of the church.1 Ananias serves as a model to the church in terms of how to welcome an enemy who comes to faith.2
17 So Ananias departed and entered the house. And laying his hands on him he said, “Brother Saul, the Lord Jesus who appeared to you on the road by which you came has sent me so that you may regain your sight and be filled with the Holy Spirit.”