Incest is very taboo and rightly so. Why? Primarily because it creates mutations in any resulting offspring. That's why many breeds of dogs have health and other issues; because in order to strengthen and solidify the desired characteristic traits of the breed being created, breeders would often breed dogs that were directly related (i.e. son-mother, father-daughter, brother-sister, etc...).
Over the years, I have often wondered why this is the case. Why children of close relatives are subject to fairly dramatic mutations as opposed to children of unrelated people, in which mutations are comparatively rare. Why this curse on relatives "keeping it in the family"? Why does "mother nature" care; for clearly this is a defined distinction and thus has some purpose.
Like so many things, if not all things, the purpose is survival. It is a function of population. e.g. Planet E is populated with the "Hunans". If they are a successful "life design" they will prosper and their numbers will grow - bringing more & more diversity within the defining limits of what makes a Hunan - making mutations a rare occurrence. On the other hand, had the life design of Hunans not been a successful one, their numbers would have decreased over time, in route to extinction. But life is not just a quitter like that.
This is where incest comes in. The rapid and dramatic mutations are an organism/species's final defense against extinction. It's sort of a last attempt to alter the species design into a more viable and successful form. As the population shrinks, it becomes increasingly likely that sexual relations will occur between ever-closer relatives. The idea being that one or more of the resulting mutations will eliminate or compensate for whatever deficiency exists in the current design; which is clearly in the process of failing as evidenced by the dwindling population.
There you have it. This is why (in terms of how is it adaptive for the organism) mutations occur in the offspring of incestuous partners.