Incest can be moral, but practically speaking it is hard to imagine such a situation. First, pregnancy has to be prevented, because the risk of severe genetic defects is very high. (Maybe there is a harder philosophical question here than it seems to me, but it seems to me that it is forbidden to bring a child into the world with such statistics.) A relationship between a parent and child is certainly outside the discussion for many reasons, among them the inability to ensure that there are no dependency relations, psychological pressure, and harm to the parent-child dynamic that a child needs.
With siblings, the above problems and others still exist, of course to a lesser degree. One can also discuss harm to other family members who are not involved in the relationship, and perhaps make weaker claims about the very existence of the taboo and the accepted social norm, and against dismantling traditional family values (which is a weaker claim, since they could have argued—and did argue—the same about homosexuality).
P.S. Regarding child adoption, it is not far-fetched to argue against it, including with regard to LGBT families (painful as that may be), because the issue is not the good of the couple but the good of the child. Once, following some discussion, I looked into studies on children in LGBT families and did not find completely reliable information. The studies I read were carried out by organizations with agendas on both sides and did not meet the standards of scientific research. In any case, the burden of proof is on the modern families, because the Welfare Ministry (the Child Services Department) has to find the most ideal parents possible for the child. Of course, I am not claiming that they are not ideal, only that there is no information. If they had brought a child into the world naturally, obviously the child would not be taken from them, and here the opposite burden of proof is on the authorities.)