TwistedMac said:
"why not siblings/father-daughter etc"
because the kids from these more often than not will have grave dissabilities of one sort or the other.. and that's just cruel to the kid.
Actually, this is a common misconception.
Incest is prohibited on moral grounds, the biological reasons are minor. Personally, I see this kind of reasoning as leading in the direction of Eugenics as the Nazis enforced it. You forbid people to procreate for reasons of the "health" of future generations.
The probability for genetic disorders is of course bigger in incestuous relations (than in most other relations of healthy adults), but it's normally highly exaggerated. Usually you'd need several generations of inbreeding to see negative results.
From Wikipedia:
"Incest may be a form of inbreeding, and some have suggested that the incest taboo is meant to reduce the chances of congenital birth-defects that can result from inbreeding. Scientists have generally rejected this as an explanation for the incest taboo for two reasons.
[...]
If an individual has an allele linked to a congenital birth-defect, it is likely that close relatives also have this allele; a homozygote would express the congenital birth defect. If an individual does not have such an allele, a homozygote would be healthy. Thus, the frequency of a defect-carrying gene in a population may go up, or down, when inbreeding occurs. The overall effect of inbreeding depends on the size of the population."
Your argument of "grave disabilities" holds more water for relationships of (related or non-related) bearers of genetic deficiencies, let's say haemophiliacs. I think, if a male bleeder procreates with a female carrier of the gene defect, there is a 25% probality (didn't look this up, just from memory) that the child is also a bleeder.
According to the logic of probability of disabled offspring, should carriers of that defect then be prohibited to marry? This question counts for other genetic deficiencies as well.