[i]As an afterthought though, what strikes me as funny, is in the beginning, brothers and sisters obviously HAD to marry one another, because there weren't any other people. I mean, how did those people after Cain and Abel marry anyone if they were originally produced by Adam and Eve? There weren't more people.[/i]

You're in fairy tale land there. Adam and Eve didn't have any daughters - at least that were mentioned in the Grimmest Storybook. Exodus is totally inconsistent there. Why did God put his mark on Cain to protect him from the wrath of other men? What other men?

Stupid inconsistent blather.

More likely scenario:

In the beginning, monkeys flew out of God's butt.