As for Cain and Able, they weren't Adam and Eve's only children. They probably "married" their sisters.
Yes, probably... But there you are reading something into the Bible. The fact is that we don't know, and there are a lot of things we don't know.
I believe that it is all literally true, but what is the literal truth? It is the literal truth that Christ came from Bethlehem, as prophesied, but the Priests believed that He came from Nazareth, and therefore ruled Him out as the Messiah. If you get stuck on nonessential details, you miss the big picture, and you may be wrong in the final analysis.
For example, suppose that Adam and Eve were themselves created directly by God, but there were humans who evolved (and of course, God started that process as well), and those evolved humans ultimately reproduced with Cain and Able to produce modern humans?
Both could be literally true.
Of course, I don't know which is true, but to fight WWIII over this issue seems counter productive to me. It would have been a lot less destructive if the Mideival Popes had simply recognized that the Copernican solar system was not inconsistent with the Bible from the outset, and refrained from making it an issue.
Do you believe "God's image" is physical?