Notice Genesis 5:4: “After he begot Seth, the days of Adam were eight hundred years; and he begot sons and daughters.” Obviously Cain married one of his sisters — one of Adam’s daughters — and Seth, Cain’s brother, did likewise.
Adam and Eve, as God proposed, were fruitful (Genesis 1:28). In today’s world, when many couples are having no more than one or two children, it’s hard for us to grasp how many children Adam and Eve probably had during their great span of life, nearly a thousand years. Adam lived almost one sixth of all the time from his creation until now.
It was not wrong to marry a sister or a brother in the beginning — no physical harm would result. More than 2,000 years later, in the days of Abraham, a man could still marry a half sister. It was not until the days of Moses that God forbade brothers to marry their half sisters (Leviticus 18:6, 11).
In pre-Flood days, when people lived for centuries, they did not age as we do today. They were able to continue bearing children, undoubtedly, for hundreds of years. After the Flood, because of living contrary to God’s laws, the human life span became greatly shortened.
Source: The Good News, August 1985