While Cain and Abel were among Adam and Eve’s first children, they were not their only children. Adam and Eve had many other children, and their children had children, etc. In Genesis 5:4 a statement sums up the life of Adam and Eve—“And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters.” Adam lived 930 years.
The fact that Cain was scared for his own life after he killed Abel (Genesis 4:14) indicates that there were many other children and perhaps even grandchildren of Adam and Eve already living at that time. So, Cain’s wife (Genesis 4:17) was a probably a daughter or granddaughter of Adam and Eve. 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” (Acts 17:26).
Many people immediately reject the conclusion that Adam and Eve’s sons and daughters married each other by appealing to the law against brother-sister intermarriage. But the law forbidding marriage between close relatives was not given until the time of Moses (Leviticus 18-20). In the very beginning of earth’s history, there was no disobedience to God’s law originally when close relatives (even brothers and sisters) married each other. Abraham married his half-sister (Genesis 20:12). God blessed this union to produce the Hebrew people through Isaac and Jacob.
But later on at the time of Moses, God gave him the laws that forbade such marriages due to the accumulated deformity of genes. Adam and Eve did not have any genetic defects, and that enabled them and the first few generations of their descendants to have a far better genetic pool than we do now. Today the marriage of people to close family members results in genetic abnormalities because there is a high risk of their recessive characteristics becoming dominant. However, when people from different families have children, it is highly unlikely that both parents will carry the same recessive traits.
In His service,