I cheated a little on this one. I actually learned the answer awhile back. However, as a kid, this one threw me.
Most recall the story, but for those that do not, let's state it before we answer the question.
We read in Genesis chapter 4 verse one and two where Eve had Cain and Abel. With these two sons, the story gives us the first murder that ever took place. It was done out of one being jealous of the other. In verse 8 of chapter 4, we see Cain 'rose against his brother and killed him'. Basically, he jumped him in a field. Later on in verse 9 'the Lord' asked Cain 'where is your brother'. So we have a couple of things to point out already. Prior to the first murder (the ten commandments have not been given to the people yet), God walked and talked with the people of Earth. Note 'the Lord' also asked where Abel was. This indicates God also was on a ground level with Abel. Scholars believe this to be Jesus, prior of being begotten.
Irregardless, Cain spouted back, 'I don't know. Am I my brothers keeper'. This would be the same as a kid telling a parent, 'What are you asking me for'. So God then tells Cain his 'brothers blood cries out to him from the ground' in verse 10. So again, God already knew, but asked Cain anyway. Now we see Cain as a murderer and liar. For punishment, God cursed Cain from the earth and made him a fugitive.
Note verse 14. Cain replies 'anyone who finds me will kill me'. This is the first indication of more people than he, abel, adam and eve on the planet. In verse 15 God puts a mark on Cain lest anyone finding him should kill him. This re-emphasizes there are more than the 4 of them. Later on in Genesis chapter 4 verse 16 and 17 we see Cain went to Nod, on the east of Eden and found a wife. Now we come to the question.
Who is this wife and these people Cain feared would kill him?
First I would like to point out some things. It's all about context.
1) In Genesis chapter 3 verse 20, we are told Eve is the mother of all living. So from this text, any human being is a off-spring of Eve. With that said, we simply are all related. Please note, there is only one human race.
2) In Genesis chapter 5 verse 4, we read Adam lived 800 years and begot sons and daughters. Note that Adam actually lived 930 years. During his 800 years of life, he had the sons and daughters. Hmm, I have seen parents having up to 12 kids in a single marriage, lasting no more than 40 years. Now, if one lives 800 years, perfect in body and health, how many kids can you really have?
So given the two above references, the answer is already before us. Since Eve is the mother of all living and Adam had many (plural) sons and daughters, the other people would have to be Cain's relatives.
OK, today that would be considered incest and quite frankly, disgusting. However, again we need to view this in context. A)God made Adam and Eve 'very good' meaning they were well and above any supermodel or athelete we have today. B)Similiar to God's commandment to Noah, Adam and Eve was multipling the earth. C) Did you also note Nod is on the East of Eden. This indicates the location of Nod was not very far from where Cain was currently located.
Also know, the laws of God was not provided till further down the line after Noah. You did not even have the concept of incest at this current time of discussion.
So there you have it. Yes, Cain was one of the first generation under Adam and Eve. His wife in Nod would most likely be a niece. A relative for sure. The other people were simply his relatives.
If you have any question you would like me to research and answer. Fire away.