Adam asked, "Do you know where your brother is?"
"No, I don't," said Cal.
"Weren't you with him at all?"
"He hasn't been home for two nights. Where is he?"
"How do I know?" said Cal. "Am I supposed to look after him?" (51.1.91-96)
We're about to get all King James on you. Check it:
And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? (Gen. 4:9)
Okay, so aside from nixing quotation marks and being in seventeenth-century prose, this quote from the Bible is almost exactly like Cal's question at the end of the excerpt. My brother's keeper is one of those phrases that have trickled down into our culture without us really remembering where they come from. In fact, in the 1955 movie version of East of Eden, they have Cal just say it outright.