God asks a question He already knows he answer to, just like He did to Adam-“Where are you?” “Who told you were naked?” “Have you eaten of the tree?” God knew the answers but He wanted to elicit personal responsibility, accountability and opportunity for repentance.
Twice God refers to Abel as Cain’s brother, thus making him understand that Abel is his own blood, his mother’s son.
Cain said it was not he responsibility to be his brother’s keeper.
Keeper means “guardian.”
Cain claims he had no responsibility to protect and guard his brother all the while hiding the fact that he had brutally murdered him.