The Bible records the fact of Adam and Eve leaving the Garden of Eden in Genesis 3:23,24, (NIV) "So the Lord God banished him from the Garden of Eden to work the ground from which he had been taken. After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life." This was before Adam and Eve had any children.
Then in Genesis 4:1 (NIV) the Bible records, "Adam lay with his wife Eve, and she conceived and gave birth to Cain." In Genesis 5:4 (NIV) it states, "After Seth was born, Adam lived 800 years and had other sons and daughters."
Genesis 4:17 (NIV) mentions Cain and his wife having a son named Enoch. Regarding the identity of Cain's wife, she was one of the family of Adam, no doubt Cain's own sister. Obviously, the earliest inhabitants of earth had no other choice than to marry their brothers and sisters in order to fulfill the divine command, "Be fruitful and multiply."
In Genesis 3:20 we read that Eve was "the mother of all living," and in Acts 17:26 it says God "hath made of one blood all nations of men for to dewll on all the face of the earth." In Genesis 5:4 we are told that Adam had daughters as well as sons, and it is reasonable to assume that daughters were born to Adam and Eve soon after Cain and Abel. The fact that they are not mentioned by name does not prove that they did not exist.
The custom long remained in vogue as seen in Abraham's marriage to his half sister Sarah in Genesis 20:12. In that period of the world men and women had not become diseased, deformed, and mentally deficient as they are today. Because of these conditions in many families up to our present time, close intermarriage between members of the same family were later prohibited (see Leviticus 18:6-17). All descendents of Cain perished at the time of the Flood.