Well... the relationships of the Kaka clan are
A) that of a clan, not your average 'civilians' (for lack of better word to describe the MCs)
B) not as relevant as the relationships between the main characters of the series
C) still not likely to consist of siblings being incestuous (but who knows), as far as we know, they don't even need mates
D) TD was joking anyways, nothing to suggest they were 'ok' with incest regardless
E) basically animals in instinct, they act differently than 'humans', at least the non-Torakaka/Elder ones we've seen.
F)uck you for beating me to my basic points, TD
>:c