How does sibling rivalry relate to Cain and Abel?
I don’t know that the Bible is clear that there was sibling rivalry between Cain and Abel. Cain was a farmer and Abel was a shepherd (Genesis 4:2). They both brought the firstfruits of their labor to the Lord as offerings and sacrifices.
Some say that because Abel brought animals instead of the produce of the land he was fulfilling the animal sacrifices God desired. But nowhere in the text does it say that. If there was sibling rivalry, it was because got accepted Abel’s sacrifice more than Cain’s (Genesis 4:4-5).
Instead of directing his anger at himself and trying to make an offering that was pleasing to the Lord, Cain turned his anger toward his brother Abel. Even God reminds Cain that he can do well (Genesis 4:7).
Instead, in an act of rage Cain kills Abel. And for this he is cursed and must be banished. God still protects him with the sign of Cain (Genesis 4:17-18). As we see another places throughout the Bible, each person is responsible for his own sin (Ezekiel 18). Cain should have worked on finding out what pleases the Lord instead of killing his brother.