God's question to Cain put him in a sticky spot. Cain had just murdered his brother Abel, probably in a fit of jealousy and bitterness. God had accepted an offering from Abel but had rejected Cain's offering, most likely because Cain's heart did not really love God anyway. God always looks at our heart and sees why we do the things we do.
So Cain's reply to God was first of all untruthful. He said "I don't know." But then he added an insulting response: "Am I my brother's keeper?" Cain and Abel — two brothers torn apart by Cain's hatred. It makes such sad reading. And the other Bible verses which refer to Cain tell us how his hatred of his brother became a byword for wickedness right through the history of God's people. We read that among the Jewish people Cain was thought of as an example of sinfulness and one who instructed others to do the same. What a terrible legacy! And it could have been so very different.
He said "Am I my brother's keeper?" Of course he was. Each of us has a great responsibility to love and care for our own brothers, sisters and family members, and a responsibility towards other people as well. We need especially to look after those who are vulnerable and who cannot care for themselves.
Lord Jesus Christ, help me to be a true brother or sister both to my family and the wider family of God and all who need a brother's care and love.