"I have already explained why same-gendered sibling relationships are ilelgal, and will remain so." That really doesn't matter. We aren't arguing that. We know this already, but legality in and of itself is not the issue. The issue is WHY it is illegal. That's the entire point of this thread which you seem to be missing. And actually slippery slope would be more along the lines of "First siblings can marry, NOW HUNTINGTON PEOPLE CAN MARRY?!" Had he said that, it would have been a slippery slope. But he didn't. He's drawing a parallel.
Firstly, it is certainly a slippery slope, not a parallel. As noted, a person with Huntington's has a fifty percent chance of passing on said disease to their offspring. Introduce siblings who have the same genetic background, and the chances of the offspring having the disease nearly double.
Secondly, it is illegal primarily because of social morality. But unlike the discrimination that gays (a Protected Class in most areas) face, there are legitimate reasons to keep it illegal, as I also noted above and will repeat and surmise.
1. It is very difficult to determine if said sibling relationsip is indeed concensual. Intimidation and/or an inherent need to protect family members may play a factor.
2. As has been shown ad nauseum, the chances of genetic diseases and abnormalties are simply too high.
3. Where same-gendered relationships go, besides number 1 above coming into play, same-gendered siblings are not a protected class and therefor there is no discrimination in keeping their relationships illegal.