not (one's) brother's keeper

Not responsible for the actions, behavior, or whereabouts of one's close relative or friend (not necessarily a brother). Often used to express frustration that such responsibility is assumed. Of course I'm disappointed that Travis got arrested again, but I'm not my brother's keeper. How should I know where Mary is? I'm not my brother's keeper.

not your brother's keeper

You can say that you are not your brother's keeper to indicate that you do not accept responsibility for other people in any way. Part of me wants to help him, but part of me realizes I can't be my brother's keeper. Note: These expressions come from a story in the Bible. Cain has killed his brother, Abel, but tries to deny it. `And the Lord said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper?' (Genesis 4:9)