One should ask how any incestuous relationships between humans could explain what appear to be "design flaws" or evolutionary adaptation of existing structures to fit new circumstances in nonhuman creatures.

E.g., gannet nostrils:

Birds of the family Sulidae, the boobies and gannets, are diving birds, plunging from height into the water. And one of their adaptations to this is that they lack external nostrils. This makes sense: the water would otherwise get shoved up their noses on impact.

So is this intelligent design? Not exactly. For though they lack external nostrils, they have everything else that constitutes nasal airways inside their beaks -- the septum, choana etc -- it's just that the nostrils are sealed off at the outside. Having nasal airways that cannot work (since they are blind-ended) is pretty pointless design. Why bother having them at all?

Reference: Some More of God's Greatest Mistakes

The above reference has has many more such examples.

Or take the case of the panda's "thumb", which, unlike our own thumbs is made from evolutionary adaptation of the radial sesamoid bone. If a god designed the panda from scratch then one would expect a thumb like ours, rather than the panda's thumb being an adaptation of the radial sesamoid bone.

So the panda must use parts on hand and settle for an enlarged wrist bone and a somewhat clumsy, but quite workable, solution. The radial thumb is, to use Michael Ghiselin's phrase, a contraption, not a lovely contrivance. But it does its job and excites our imagination all the more because it builds on such improbable foundations.

The Panda's Peculiar Thumb by Stephen Jay Gould

But back to humans. Creationists need to claim Abel and/or Seth either mated with their mother, Eve, or any unnamed sisters Eve may have begat by Adam, if they choose to stick just to females mentioned in the Bibles accepted by most varieties of Christians today. Some will suggest that the male offspring of Adam and Eve "married" their sisters when pressed, but again you then have to envision substantial design changes in many areas of the human body due to inbreeding between the descendants of this original "perfect" couple over just a few thousand years, if you accept the creationist notion that the Earth is nor more than a few thousand years old while at the same time ignoring the fossil record.

To accept that view, one has to ignore not just the hominid fossil record which provides information on human evolution, but ignore the evidence for an earth many millions of years old (the earth is about 4.6 billion years old) found in the entire fossil record.

It seems somewhat implausible to use the incest defense of creationism as an explanation of goose bumps in humans, which appear to be relics of our evolutionary past.

Humans get goose bumps when they are cold, frightened, angry, or in awe. Many other creatures get goose bumps for the same reason, for example this is why a cat or dog’s hair stands on end and the cause behind a porcupine’s quills raising. In cold situations, the rising hair traps air between the hairs and skin, creating insulation and warmth. In response to fear, goose bumps make an animal appear larger – hopefully scaring away the enemy. Humans no longer benefit from goose bumps and they are simply left over from our past when we were not clothed and needed to scare our own natural enemies. Natural selection removed the thick hair but left behind the mechanism for controlling it.

Reference: Top 10 Signs of Evolution in Modern Man

And vestigial structures in human anatomy seem more likely to be a result of evolutionary changes rather than defects caused by inbreeding.

To believe that the earth is only a few thousand years old one has to reject radiometric dating methods, such as potassium-argon dating. To believe that the earth is only a few thousand years old, one has to discount the scientific method and the findings of astronomy, biology, chemistry, cosmology, genetics, geology, physics, zoology, and other scientific disciplines in order to maintain the cherished belief that one can understand the development of life on earth and the development of the universe solely by reading the Bible and adhering to a literal interpretation of its contents.

Not all Christians, of course, take that approach. There are denominations that have instead adopted a theistic evolution viewpoint.