Before the Bible "Cain" killed his brother "Able."<quoted text>
Morality comes from parents, not the bible. Witness a sibling punching his brother in the nose. Now, is the parent to tell the puncher to go read the bible for moral guidance? No. The parent would say to the puncher, "how would you like me to punch you in the nose so you know how it feels"? Problem solved with no bible needed.
"Able" was a good guy, he didn't even cheat in poker.
So I would say you are correct in that Adam and Eve didn't teach Cain to respect his brother. It was the first case of road rage but without the cars.