Actually if you are from a family with no defected genes, incest would lead to perfection. In the cases, where there are defected genes in a family, pairing these genes would result to a birth defect. For example, if there is a defected gene that could result to diabetes in a person, in case of insect there is a strong chance that this gene pairs up with another defected gene and result in a child suffering from diabetes. Of course, this could happen to the children of any two people whose defected genes matches; therefore there are certain genetic tests before marriage in some countries to reduce the birth defect rates. So if a family with perfect gene pattern inbreeds, there would be no hereditary problem with their future generation unless their gene pattern have a sudden change, that can happen to any specie.