When confronted by God about his brother Abel whom he had killed, Cain, living in his pseudo-reality exclaimed, "Am I my brother's keeper?"
I've heard/read an argument that anwers this, "no". But the weight of Scripture and what God calls us to as his people in an interdependent, interactive relationship of love in communion with God, seems to clearly indicate that we are certainly to be our brother and sister's keeper.
Of course this works both ways. We need to be open to other sisters and brothers helping us as we need it. And we need to be awake and alert to those around us who at least need our prayers as well as our friendship.
Is there an alarm going off in what you see in a brother or sister's life? Don't overreact of course. First pray. And depending on your relationship to that person, proceed in love to be there for them. Be slow to speak and quick to listen. Don't judge their heart, yet don't turn a blind eye to what might be destructive for them and others, in their walk in God.
This is tricky territory and not one that lends itself to answer books so that one knows exactly what to do when "A" is happening. This is where we need that dependence on God as well as that interdependence on each other as we proceed to live by faith in the will of God together, in Christ Jesus our Lord.
What might you like to add here?