Having been handed down by oral tradition for millennia, the events related were only finally written down much later, as faithfully as possible, at the command of the Prophet and God-Seer Moses.
The gene-pool must have been so large that the results of what we call incest, well-known today, simply did not operate then).In their curiosity, men have often been tempted to speculate about the gaps of the telescoped history in Genesis, in ways which have not always been healthy or profitable.For example, Darwin's then new theory of evolution clashed with the literal understanding of that part of Genesis.Inevitably, a false understanding of these chapters would enter into conflict with scientific theory - all the more so if the scientific theory were also false.After all, the conflict between Darwin's theory of evolution and the literal interpretation of Genesis affected and affects above all Protestant societies, as in nineteenth century Britain or in the present-day United States.
This is because Protestants lack a Patristic understanding of the Scriptures.If there are Christians who are happy to interpret those first chapters literally, then so be it.However, I think that they are a tiny minority among all Christians.The Genesis account of creation is a telescoped version of the essential events which took place before recorded human history.Since only the essential events are related (no doubt because the non-essential ones had been forgotten or confused by oral tradition), there are many gaps. Just because the sisters are not mentioned does not mean that they did not exist.That is, they do not understand the Scriptures spiritually, ascetically, allegorically, poetically, but only literally. This explains why Darwin's theories have posed far fewer problems in both Roman Catholic and Orthodox societies than in Protestant societies.