Cain is listed as the first child of Adam and Eve, the third human being to come into existence in the history of the world.

Gen 4:1  Now Adam knew Eve his wife, and she conceived and bore Cain, and said, “I have acquired a man from the LORD.”

Abel is listed as the second child of Adam and Eve.

Gen 4:2  Then she bore again, this time his brother Abel. Now Abel was a keeper of sheep, but Cain was a tiller of the ground.

The next several verses describe the tragic murder of Abel by Cain and Cain’s subsequent punishment.  Then in verse 17 Cain’s wife is mentioned without any introduction.

Gen 4:17  And Cain knew his wife, and she conceived and bore Enoch. …

Who was Cain’s wife?  Where did this mysterious woman come from?

In Gen 3:20 we read:

Gen 3:20  And Adam called his wife’s name Eve, because she was the mother of all living.

In other words, everyone (other than Adam and Eve) are descendants of Eve.  Thus Cain’s wife must have been a descendant of Eve, a daughter, or possibly a granddaughter.  The Bible mentions that after Adam and Eve had Cain and Abel, they had more children.  Next was another son called Seth.

Gen 4:25  And Adam knew his wife again, and she bore a son and named him Seth, “For God has appointed another seed for me instead of Abel, whom Cain killed.”

Then more sons and daughters. 

Gen 5:3  And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth.
Gen 5:4  After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.

It’s obvious that Adam and Eve’s children married each other.  Brothers married sisters.  Cain married one of his sisters, or possibly a niece.

Today, if brothers and sisters marry their children are likely to have birth defects, because having the same parents they are likely to have inherited the same defects in their DNA from their parents.  However, Adam and Eve had perfect DNA, and their children perfect or near perfect DNA, so birth defects were not an issue for the early generations of humans.  However, as more DNA defects accumulated in successive generations, birth defects did become a problem and later, at the time of Moses, God gave instructions not to marry close relatives, Leviticus 18; 20:10-21.