Truthfully, it depends on the culture. In current times, most "civilized" nations frown apon incest; but if you take a look back through histroy, royal families inbred to keep their "royal blood" "pure"(which did, however, lead to birth defects). The roman catholic church allows incest to a certain degree, ie. between second and third cousins.
Also, almost every human alive is a result of incest; meaning that at some point in everyone's family tree, inbreeding occured. If you look at it from an evolutionary standpoint, it would have been difficult to populate the earth to its current levels without relatives procreating with other relatives. If you look at it from a "black-belt" christian point of view(the ones who belive in the creation story of Adam and Eve) then incest MUST have occured to populate the land.
Basically it is up to the individual to determine wheter it is right or wrong, and for the ones who will support it being a sin using scripture, then just take a look back at Adam and Eve; whether it was right or wrong, it happened.