Ge 4:8 Now Cain talked with Abel his brother; and it came to pass, when they were in the field, that Cain rose up against Abel his brother and killed him.

9 Then the LORD said to Cain, "Where is Abel your brother?" He said, "I do not know. Am I my brother's keeper?"

10 And He said, "What have you done? The voice of your brother's blood cries out to Me from the ground.

Six times here in verses 8 to 11 the word brother is repeated over and over again. The whole thing is intentional. It is to show the seriousness of Cain's sin. Cain was not being his brother's keeper, instead, he was his brother's murderer.

Moses here emphasizes the nature of murder---since we all came from Adam and Eve,  murder then is essentially the premediated killing of a brother or a sister.

The purpose here is to reinforce to Israel the command: "Thou shall not murder."