I think that, however society should choose to treat parental incest, for the sake of consistency and coherency it could be compared to parents giving children legal but strong drugs - whether performance-enhancing or recreational.

Obviously any medical expert who's just doing a job and is afraid of being sued would "advise" against either if consulted. Clearly, parents don't have total sovereignity over their children, and most of the "decent" parents wouldn't ask for it anyway while most of the abusive parents would love it. On the other hand, clearly the vast majority of people are hostile to the idea of thorough, case-by-case state intervention, a social worker ordering parents where exactly to draw the line, etc - both for political and pragmatic reasons.

But still, there are obviously parents who would, in good faith and with good intentions, want to introduce their child to sex or certain drugs. In such cases, not only are their preferences being unfairly violated, they might be right about it being safe and worthwhile for their child. Is there any way at all to filter those benign cases out from deliberate abuse, dangerous carelessness, etc? I can't think of any.

(Discriminating against sibling incest is just as senseless and barbaric as discriminating against functioning drug users or homosexuals, IMO. Siblings should certainly be able to enter the complete, standard, state-sanctioned civil union with all its trappings - whether we rename it from "Marriage" to something else, as some liberals and libertarians propose, or not.)