difficulty only exists among those closely related. if you're once removed then the chance is barely greater than normal. first cousins also aren't really at a bigger risk - 4% higher than the norm, and 2nd cousins cut it down to 2%.
there isn't much against incest between two consenting adults, but incest is also the least comparable of them all. homosexuality is a sexual orientation. beastiality, if you have some attraction to a creature, you might be able to weasel an excuse in. but incest doesn't have anything going for it like that - if you're attracted to someone then being related isn't really a qualifier. it might make things more spicy if you're into that taboo, or it might make you gain a bit more attraction because of it, but it doesn't actually do anything in and of itself. it's basically a fetish. so while you might gain some attraction to people of the same sex or a horse on the basis of them being the same sex or them being a horse, 'sister' doesn't really amount to anything. if you're attracted to women and one is your sister it doesn't change that the main attraction, and reason it's your sister, is because you're attracted to women in the first place. to one line it: incest isn't a sexual orientation. homosexuality is.
beastiality simply runs through the matter that animals can't consent. you can argue it due to lack of proper brain functions to how we define consent, but end of the day, they can't. it's basically rape.
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The King Wang.
Listen up Urinal Cake. I already have something that tells me if I'm too drunk when I pee on it: My friends. - Colbert.