I suspect the reason sex between close relatives is generally viewed negatively in most societies (but not in all scoieties) is mainly because it weakens the gene pool. It is likely that more genetic abnormalities resulted and was seen as a bad thing to do. As to incest, the same reason likely was a factor, and obvious other reasons based on the parents role in raising their children.

incest

Here is some information on cannibalism and its history in many societies.

cannibalism