Incest in real life is always portrayed as deformed redneck hillbillies fucking each other degenerately because they’re missing some part in their brain that measures morality. Bonus points if they’re also serial killers.
In fiction, and especially in anime, incest is always portrayed as two people who grew up together and that initial platonic relationship blossomed into love. It’s cute as shit. And then the added element of taboo makes it exciting because they want to be together but can’t, or they are together but they might get caught. It’s like a higher stakes Romeo and Juliet. Seriously, look at any fictional incestuous relationship but change them to being childhood friends: it’s still a really cute love story. But the taboo part is what makes you go, “Oh no, what happens next?”