It is a general tenet at law that I am not my brother’s keeper. (source: first year property class, distinguished Stanley Johanson, professor of law). Generically, brother is anyone besides yourself. Unless acting in concert with your brother (or any other person), you are not responsible for the other person’s actions. But in all circumstances, you must answer for your own conduct.
As a Christian, the question posed, “Am I my brother’s keeper?” may be answered differently. Dr. Warren Wiersbe answers the question emphatically: “The answer of course is ‘Yes!’ The two great commandments are to love the Lord and to love your neighbor, and our neighbor is anybody who needs our help. Luke 10:25-37. As members of the human family, we should care for one another, and as members of God’s family, we should love and serve one another. Gal. 5:13” (source: Old Testament Words for Today, page 11, 2013 by Warren W. Wiersbe)