Consider the law of the Kinsman Redeemer:
If brothers dwell together, and one of them dies and has no son, the widow of the dead man shall not be married to a stranger outside the family; her husband’s brother shall go in to her, take her as his wife, and perform the duty of a husband’s brother to her. And it shall be that the firstborn son which she bears will succeed to the name of his dead brother, that his name may not be blotted out of Israel.
Deuteronomy 25:5-6
This law defines how to be a “brother’s keeper.” Perhaps we’re told that Cain left and knew his wife (Genesis 4:16-17) specifically because he wouldn’t marry Abel’s wife in order to continue his dead brother’s name.
Neither Cain nor Abel
Eve assumes God will use Cain – she says as much when she first speaks. And we, seeing Abel’s sacrifice, assume God will use the younger son to fulfill the blessing of “be fruitful and multiply.” But our assumptions are dashed by murder. Cain leaves the scene, and Abel dies. What will God do to solve this problem?
Perhaps the right name of the story should be “Not Cain, not Abel, but through Seth.”
Seth’s name means “appointed.” As in, selected by God to be used for a specific purpose. God’s purpose.
This is the nature of things.