One thing that doesn't need speculation is that Cain and Abel had brothers and sisters. (Gen. 5:4) The "process of time" mentioned here easily allows others to have been born. Of course, we aren't told how time passes here, except that it was less than 130 years; the age of Adam when Seth was born. (Gen. 5:3) While we are told later that Seth replaced Abel in the bloodline, that doesn't mean he was born right after the others.
Ruth 4:17 and 4:22 are perfect illustrations of this. Jesse begat David - but David was the youngest of a whole brood of boys. it doesn't matter that the others were older. What matters is that God doesn't look at people like we do. "Man looketh on the outward appearance, but God looketh upon the heart." (1 Sam. 16:7) Who is to say that the murder of Abel didn't set sibling against sibling, with God choosing to appoint a child not yet born, who had not become part of the fighting? It is entirely possible. It's also possible they were younger, and Adam and Eve had to wait for the right one.
Getting back to the sure things of God's Word, we see that Cain brought an offering of grain. That, of course, could have been proper - but, notice that Numbers 18:12 says it shall be the firstfruits. Whether Cain brought the best is unclear.
We do know he brought an offering unto the Lord. One's offering should be the best, becuase He has give us his best; it is our reasonable service. (Rom. 12:2) Even if it was, however, it must be the right kind of an offering for a specific time, as that verse in Leviticus shows. Only blood can bring about the remission of sin.
Finally, moving away fromt he specific sacrifices made by Cain and Abel, we see that a system of sacrifices had already been put in place. We're not sure what it was. However, the important thing is that it showed the continued relationship between God and mankind. The first sacrifice - made by God Himself to make clothes for Adam and Eve - would be followed by many, many sacrifices, all of which could only cover sin. These were pictures of what Christ would do one day on the cross, when - as the Lamb of God - he was slain for the sins of all mankind. Even in the Millennial Kingdom, there will be sacrifices, but these will only be for a memorial, like the Lord's Supper is today. There is no more need for sacrifices now. The curse of sin is broken forever, thanks to Jesus Christ's death and resurrection! Hallelujah!
