As Pilar's key line of dialogue alluded to, the reason incest is illegal is because inbreeding has such a high rate of birth defects.
The idea of mating with your own sibling is abhorrent to most people on a sort of instinctive level. No matter how objectively "hot" your sister/brother is, a 'normal' mentally healthy person won't be able to feel any attraction toward them. It's probably tied to evolution. Over millions of years, people who mated with unrelated strangers have had much better odds at producing healthy hearty offspring and thus promoted their own bloodlines... as opposed to inbreeders who too often produced severely effed up ree-rees who never procreated.
But one way or another you have to be aware that you're related. If two half-siblings aren't aware they're such, and there's no obvious physical resemblance staring them in the face, and so as far as they know they're unrelated, well, what's stopping them from being attracted to each other? And if they've been in love with each other for over 20 years, suddenly finding out after that long might not be enough to change their feelings.
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