It's a memorable quote in the bible found in Genesis 4:9, meaning:
1. Are you responsible for how you respond to your brother?
2. Are you held accountable for how you treat your fellow man?
It's really a sarcastic question/remark from Cain to God after Cain killed his brother, Abel, to keep from admitting to the fact that he did kill Abel.
Nino Brown: To CMB. On and on. Am I my brother's keeper?
CMB: Yes I am.
Nino Brown (louder): Am I my brother's keeper?!
CMB (louder): Yes I am.!
Nino Brown (louder): Am I...my brother's keeper?!!!
CMB (louder): Yes...I...am.!!!
The Duh Duh Man (studdering): Yes.......I am.!!!
-from NEW JACK CITY (1991)