The incest is bad, and the main reason for that is that more often than not there's bad genetic mutation, thus there's very large portion of the offspring that is either with mental anomalies, or with physical anomalies.
If we go by the Biblical version of how the first people were created and that from them rose all others over time, means that the incest would be a must for such thing to happen. Biologically speaking, if that really happened, the human kind would have not developed as it has, as the gene pool would have become very deteriorated, with individuals with very low intelligence, and physical anomalies dominating. Over time, that would have probably led to the extinction of the human kind rather than becoming large and stronger, as the population would have been very prone to all sorts of diseases, not capable of proper thinking, not being able to develop, and simply hardly finding it possible to survive.
