Phrase(s): I am not my brother’s keeper. [and] Am I my brother’s keeper?
Prov. You are not responsible for another person’s doings or whereabouts. (Biblical.) • Fred: Where’s Robert? Jane: Am I my brother’s keeper? • Jill: How could you let Jane run off like that? Alan: I’m not my brother’s keeper.
McGrawhill's American Idioms And Phrasal Verbs