This verse begins by stating she "again bare Abel." I'm not sure why some believe Cain and Abel were twins, but it is certainly possible. In populating the Earth, it would be very helpful to have many multiples. On the other hand, Seth is singular. Yes, he could have been singled out for God's purpose, at first. Still, whatever tradition states that they were twins can't really be proven one way or the other.
Abel was a keeper of sheep, whichmeans tender of them. In verse 20 of this chapter, we read that one of Cain's line, Jabal, was the father - that is, the first - of those who had cattle. This seems to refer to the roving, cowboy-type who takes his cattle over vast areas, as he also was the father of those who dwell in tents. It could also refer to animal husbandry, the breeding of them for specific purposes. Either way, this is different from Abel.
Abel's interest was in caring for sheep. He'd heard of the animal slain by the Lord as the first sacrifice - one of many, which would lead to the Lamb of God, who taketh away the sin of the world. (John 1:29) Abel would be able not only to clothe people, but to provide that for which one could repeat what was done for his parents.
Abel, then, didn't just "guess right." There's a reason Abel chose the sacrifice he did later. It's one which Cain ignored. We can't see into their minds to know for sure. But, we know God's Word. He says that without the shedding of blood, there is no remission of sin. (Lev. 17:11)
Cain, on the other hand, was a farmer. As that incredible, new world, which held so much promise, turned on its axis, and went around the sun, Cain would see how wonderfully the ground produced food; even if there were thorns and thistles, too. Abel, meanwhile, could discuss the wonders of animals.
One imagines Cain and Abel's dinnertime conversations must have been fascinating, as they learned about the world. Cain, after all, tilled the ground - it doesn't say what kinds of seeds, so he must have seen so many interesting ones. Abel, on the other hand, kept sheep. But, he would have seen so many other unique things in the process. Including, perhaps, wolves killing one of the flock. It isn't known how quickly some aniamls became meat eaters, though they were originally all plant eaters. (Gen. 1:30) They may, however, have become meat eaters rather quickly.
