We're from the mindset that sex is an option in all relationships, especially close ones. Granted that there is a natural, psychological aversion to feeling romantically attracted to your own siblings or litter mates, it doesn't stop curiosity, exploration and experimentation. Not in any species. My sister and I pretty much explored the heck out of each other growing up, initiated by one or the other for years. Practicing, we called it. We weren't "dating," had no intention of carrying on forever and stopped just as soon as we became preoccupied with our own external relationships.
As for what's so bad or not, *I* grew up thinking all sex was "naughty." It was an unavoidable sin I was ashamed of. So I didn't get over it until middle age.
My wife on the other hand, she grew up feeling that sex was a natural, healthy, fun, exciting, pleasureful, innocent way to spice up anything. And she eventually got me to recognize the same. So... she's *still* having sex now and then with a couple cousins. And as for animals, it's an "availability" left up to the animal. If our dog wants to play ball instead of get balled, then that's what we do. If the dog keeps jabbing its noise in her twat, she's probably going to permit/invite more.
If her cousin hasn't had a blowjob and is just randy as hell, she takes delight in "helping him out." And vice versa. The idea of being "naughty," doing something that is dangerous, in that there would be a nuclear meltdown if family caught on to it, gets her really, really horny.
Would I "go with" my sister today? Hmmm.... I suppose. I don't think of it, but I sure, I think I would. Just that it doesn't occur to me to. She is happy with her husband. He'd probably have great difficulty with it. It might cause her a lot of grief. Not that I'm against it. Just not something I think to do.
Short answer:
To us? No difference. Sex is sex. And it depends totally upon the two (or more) individuals engaged in it whether it is okay or not.
"Nothing is either good or bad, but thinking make it so."
Incest is not "worse than" zoophilia. It's just other than.