First we have to know that everybody came from Adam & Eve - And He has made from one blood every nation of men to dwell on all the face of the earth,” Acts 17:26. The gospel makes sense only on the basis that all humans alive and all that have ever lived are descendants of the first man Adam.  Thus, there was only one man at the beginning—made from the dust of the earth (Genesis 2:7).

This also means that Cain’s wife had to be a descendant of Adam. She couldn’t have come from another race of people and must be a child born from Adam & Eve.
Genesis 3:20  Says “And Adam called his wife’s name Eve, because she was the mother of all living.” In other words, all people other than Adam are descendants of Eve Because she was the only woman.

Cain was the first child of Adam and Eve recorded in Scripture (Genesis 4:1). He and his brothers, Abel (Genesis 4:2) and Seth (Genesis 4:25), were part of the first generation of children ever born.

Even though the Bible specifically mentions these three brothers. Adam and Eve had other children.
Genesis 5:4  Says “After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.”
The Bible does not tell us how many children were born to Adam and Eve, but considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem logical to suggest they had many Children.

Remember, God commanded to (be fruitful, and multiply) (Genesis 1:28).

Then Cain went out from the presence of the LORD and dwelt in the land of Nod on the east of Eden. And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son—Enoch.
From this context we gather that Cain had married a wife before he ever slew his brother. 
Many people think that Cain wasn't Marries when he killed his brother, but this was not the case.

So in conclusion Cain was married before he went to the land of Nod. 
He didn’t find a wife there but “knew” (had sexual relations with) his wife.

Now this makes sense because of what Nod means. Nod means “wandering” in Hebrew. So when Cain went to the land of Nod, he was literally going to the land of wandering, not a place full of people.

Ok so whats next ?

• What become of Cains Family?
• Why did God let all of this happen?

If you haven't excepted God's calling yet - Don't wait till tomorrow - do it today.  God has called everyone to come and know him personally.  He sacrificed himself so we could have a chance to know him.  Times are getting worse and worse - so don't delay and take a chance at not knowing Jesus.