Incest was essential since Adam and Eve and their children were the only humans at that time. Here is an excerpt from the website answersingenesis.org that explains it further.

The Bible teaches clearly that all human beings are descendants of Adam and Eve-thus there had to be inter-marriage between brother and sister. There would have been no risk of this causing harmful deformities in the offspring, because mutations (accidental changes in the hereditary information) need time to occur and accumulate in a population. Thus the first few generations would have inherited perfect or near-perfect genes, largely undamaged by mutations. In the pre-Flood world, harmful radiation from the sun and other cosmic sources was considerably more filtered than it is in the post-Flood world. Increased radiation and depletion of oxygen in the atmosphere (subsequent to the lowering of the atmospheric pressure) may have contributed greatly to man's rapid degeneration. With the passing of many centuries, many of these harmful, degenerative changes became added to the human race, so by the time of Moses it was absolutely necessary to have laws against incest, and these were given to mankind (Leviticus 18-20). Today there would be even more chance of deformity/disease in the offspring of such a union than in Moses's time. Because of the long life-spans, Adam and Eve's descendants may have been very numerous and widely dispersed before Cain even took a wife-that is, any inhabitants of the land of Nod would have been descendants of Adam and Eve, who had many sons and daughters.