Perhaps Abel was married.
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.