Pre-post edit: I apologize for going full autistic here but I like talking about biology &c.

Interesting that you had those ready to go, tho I’ve done a bit of a deep dive of the topic myself to try and understand it (IIRC I was spurred by an unfortunate series of back-to-back viewings of Elfen Lied, Attack on Titan, and Sword Art Online, which turned me off of anime for a year) so I kinda get it. I think maybe saying “siblings get purposefully disgusted” sounds more intentional than you meant, tho.

Interestingly enough, incest in general is not quite so damaging across the animal kingdom (or plants, but plants are a whole other discussion) as many people think. A healthy population should have enough genetic diversity and lack of those garbage genes which cause defects that an incest pairing won’t affect most offspring. Humans are just much more closely related to each other than would be usual for a population our size – we’ve gone through at least one (possibly more than one) population bottleneck that left us less genetically diverse than we should be – so it’s a lot easier for genetic diseases to be passed on through incest.

In some cases, incest has allowed animals to colonize environments they’d otherwise not be able to. Komodo dragons might be one such example, as they use ZW chromosomes instead of XY – females are ZW while males are ZZ – and are capable of parthenogenesis. That means a singular female can, separated from any male, lay a clutch of “clone” eggs which will all hatch male and thus have a mate to continue the species. This may have been how Komodo dragons colonized the islands they call home.