As I continue my journey through Genesis I find myself asking a new question. I understand Adam and Eve. I get that they had two sons, Cain and Abel, but after Cain killed his brother he was cast over to another part of the land. How did Cain meet another woman? I thought God created just Adam and Eve. Did he ultimately create other human beings in order for Cain to reproduce and his children?
We don't immediately reach the correct answer because it is so ingrained in us that marriage between brothers and sisters is wrong. But it is wrong because the world is decaying and close relations spread genetic diseases. In the beginning, the world was perfect. After all, since Eve was made from Adam's rib, you could say that Adam married a modified clone of himself.
"And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters" (Genesis 5:3-4). We don't know how many children Adam and Eve had, but they had at least five sons and two daughters. Given the time frame, they could have had quite a few more. So Cain likely married one of his sisters.
As time goes on, this falls out of practice. By Abraham's time, he is married to his half-sister, but it appears to be an uncommon practice. Both Pharaoh and Abimelech assume that because Sarai was Abram's sister that they weren't married. By Moses' time marriage between close relatives is forbidden. In our day even the marriage of cousins is frowned upon.
Since you are working through Genesis, let me recommend a study guide: Genesis.