Et ait Dominus ad Cain: «Ubi est Abel frater tuus?».
Qui respondit: «Nescio. Num custos fratris mei sum ego?».
That word custos came up earlier in Genesis 2:15.
Tulit ergo Dominus Deus hominem et posuit eum in paradiso Eden,
ut operaretur et custodiret illum.
This verse tells us that "The LORD God took the man and put him in the garden of Eden to till it and keep it." The Hebrew word used in both of these cases is shamar. Strong's Hebrew dictionary defines shamar thus:
properly, to hedge about (as with thorns), i.e. guard;Jeff Cavins, in the Great Adventure Bible Study, points out that God put Adam in the Garden of Eden to till it and guard it. What did it need guarding from? Among other things, the serpent, that nachash of Genesis 3. Surely this serpent was a dangerous creature which posed a threat to Adam, his wife Eve, and the whole garden. This duty of guarding, of keeping, this obligation to shamar, would involve Adam laying his life down for the safety of his wife... unfortunately, Adam did not do his duty. We know what happened next.
generally, to protect, attend to, etc. — beward, be circumspect, take heed (to self), keep(-er, self), mark, look narrowly, observe, preserve, regard, reserve, save (self), sure, (that lay) wait (for), watch(-man).
Cain was supposed to be his brother's keeper. This duty to protect and preserve his brother should have involved laying down his life for Abel, but instead Cain took Abel's life.
Christ was truly his brother's keeper. Christ laid down his life for each of his brothers and sisters. And that is what we are called to do, if we want to be conformed to Christ.