(Gen. 4:16-17)--"Then Cain went out from the presence of the Lord, and settled in the land of Nod, east of Eden. 17And Cain had relations with his wife and she conceived, and gave birth to Enoch; and he built a city, and called the name of the city Enoch, after the name of his son." (NASB).
We see in the Bible in Genesis 3 where Adam and Eve were cursed and sent out of the Garden of Eden. In Genesis 4, Cain kills Abel. In Genesis 4:17 above we see that Cain had relations with his wife. Where did he get his wife? The answer is simple. Cain married one of his sisters.
Genesis 5:4 says, "Then the days of Adam after he became the father of Seth were eight hundred years, and he had other sons and daughters." Leviticus 18:6 says, "No one is to approach any close relative to have sexual relations. I am the LORD." We see that Adam and Eve had many sons and daughters. The genetic lineage of Adam and Eve was perfect so marrying a sister wasn't going to cause birth defects. It wasn't until much later, during the time of Moses, that incest was forbidden as the genetic pool became less and less able to stand interbreeding.