Speaking from the point of view of someone who enjoys incest :v, to put it simply, I like it, I just like it, I don't know why :v I remember that one day, I took a look at all the things that I liked to read, and realized the only common point between them was incest :v Then I begin reading incest stuffs, and I can be sure that I really like incest. It just came naturally, and I think that's how personal taste works.
People often list some reasons why incest is interesting, such as: the forbidden love, the unique and close relation between family members, Oedipus complex, blah blah blah... I don't know about others, but in my case (as you can see), there's no logical reason behind my fetish. I just realize that I like it, that's it. I gets excited when there's a brother character in a manga, anime, or game. And the plotwist which reveals that two strangers are related usually makes me happy, not disgusting :v
On the other hand, I feel put off when the two "siblings" are revealed that they're unrelated. To be honest, I detest this kind of plotwist, and I think it's a half ass one. If you can't stand incest, why would you ship two siblings and hope that they're unrelated? If you want unrelated shit, go read normal stuffs. Mah, I know that there are some people who feel relieved when they're unrelated, and they think that step siblings are the best because they got the forbidden love between siblings, but at the same time they're not related. Best of both world, right? Boooooooooo, but that's also fine, not that I care about them :v
To put it simply, incest is like this :v People who like it will like it, people who don't like it won't. And the two sides will struggle to understand the others' opinion :v
Btw, this image is basically my life motto. Everyone's has different opinion, and none of them is wrong.