‘Now I would have you know, brothers and sister, that the things which happened to me have fallen out rather to the progress of the gospel,'
Paul wants to prevent the Philippians from becoming discouraged at the thought of what has happened to him, so he assures them that what has happened to him, rather than being a hindrance, is actually furthering the Gospel of Christ. And yet even he would have had no idea of the fact that what he wrote from prison would become of such value to so many throughout the centuries. How much we would have lost if he had not been put in prison. Many wondered what God was doing. We know what He was doing.
The translation ‘brothers and sisters' recognises that adelphoi was intended to include both. Indeed the sisters were quite prominent in the Philippian church (see Philippians 4:2).