After destroying a quarter of the entire earth’s population by murdering his only brother, God asks Cain, “Where is Abel your brother?”
Cain responds with a question in return: “I do not know; am I my brother’s keeper?” We often hear that Cain was the first murderer in the Bible. I might argue that we could conclude his dad gets that honor. God told Adam, “eat… and die” and Adam let his wife eat, knowing what God had said. In that sense, Cain did not start anything new but continued in sin as his father before him. And, just to clarify, that was not the only sin. He lied to God. His anger was sinful. That also probably led to coveting of what Abel had (sheep). He also very likely stole the sheep after Abel died. He certainly dishonored his parents by murdering their child. Not to mention his failing to love God. Cain did not just murder.
“Am I my brother’s keeper?” God answers “What have you done?” God tells Cain that his brother’s blood is crying to God from the ground and then curses Cain. On one hand, God is our keeper – ultimately, each person’s lives are in His hand. We just cannot be responsible for everyone’s safety all of the time (e.g. parents must entrust their older children into God’s keeping). But, on that other hand, we are our brother’s keeper. God’s answer to Cain was “Yes!” For our brothers and sisters in Christ, we must keep them by, first, not sinning against them. This includes sins of the tongue. Second, we must keep one another … with a “unity of mind, sympathy, brotherly love, a tender heart, and a humble mind” (1 Pet. 3:8). We love one another.