I don't either, other than environment, after some thinking and collaboration of thoughts here.

I meant to separate incest only as relationship and pairing vs simply teaching or mutual pleasure. I didn't mean to exclude parental incest from either just that parental incest in the case of forming a pairing relationship would not be ideal. Singling incest I didn't mean to exclude.

Honestly the issue always comes down to how the family raises the child to view others. Do they automatically obey because of age or familial relation? Is that the main lesson? Or are they taught that they don't have to obey or associate with those they don't like or who hurt them. Are they raised with autonomy and bodily sovereignty or not? Are their opinions respected. Those seem like the most pertinent questions.