brother
be your brother's keeper be responsible for the behaviour of a relative, friend, or associate.
☞ The phrase originated in the Bible (Genesis 4:9): 'And the Lord said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper?'
Big Brother the state perceived as a sinister force supervising citizens' lives.
☞ The expression comes from the slogan Big Brother is watching you in George Orwell's novel 1984.