If you believe the first 2 chapters to be chronologically correct, then even if chapter two is a retelling of the 6th day, the chronology of chapter 4 puts Cain as offspring #1, and Abel as offspring #2, then the murder of Able, then the expulsion of Cain, then the marriage of Cain, and THEN the birth of Seth to replace Able (lends to the theory that the 2nd (and subsequent) chapter is to describe the uniqueness of the lineage of the Jews and then Christ), then after all of that, the birth of other sons and daughters to Adam and Eve in chapter 5. Chapter 5 is interesting because it again goes back over the timeline of Adam, stating that Adam was 130 when Seth was born, Seth being the first son of birthright (remember a Jew is telling this story so that lineage is important, who is the first son..), then after Seth is the first mention of the seemingly subsequent sons and daughters. Now, in my limited understanding of the early Hebrew culture, then the only possibility of children before Cain/Abel/Seth would have been girls. So that leaves room for Cain to have married a sister, but not multiple generations in my mind seeing as how Cain would be the first (albeit disinherited) son. So if there were only sisters born prior to the first-born sons, then who were those that Cain feared would kill him for his sin, prior to the birth of the sons and daughters? I'm just throwing this out there for discussion, I'm not trying to convince anyone that I'm right, I just like the discussion.