Incest laws in general makes no sense.

I believe I've previously pointed out inbreeding depression to you; from Jensen 1969:

In Japan approximately 5% of all marriages are between cousins. Schull and Neel studied the offspring of marriages of first cousins, first cousins once removed, and second cousins. The parents were statistically matched with a control group of unrelated parents for age and socioeconomic factors. Children from the cousin marriages and the control children from unrelated parents (total N=2111) were given the Japanese version of the Wechsler Intelligence Scale for Children (WISC). The degree of consanguinity represented by the cousin marriages in this study had the effect of depressing WISC IQs by an average of 7.4%, making the mean of the inbred group nearly 8 IQ points lower than the mean of the control group. Assuming normal distributions of IQ, the effect is shown in Figure 9, and illustrates the point that the most drastic consequences of group mean differences are to be seen in the tails of the distributions. In the same study, a similar depressing effect was found for other polygenic characteristics such as several anthropometric and dental variables.

The mating of relatives closer than cousins can produce a markedly greater reduction in offspring's IQs. Lindzey (1967) has reported that almost half of a group of children born to so-called nuclear incest matings (brother-sister or father-daughter) could not be placed for adoption because of mental retardation and other severe defects which had a relatively low incidence among the offspring of unrelated parents who were matched with the incestuous parents in intelligence, socioeconomic status, age, weight, and stature.

No doubt one could dig up more recent results with a suitable search.