In this case, the answer is: because, in my opinion, the developers themselves are playing with this fetish and repeatedly dangling this carrot in front of the players.
Well, in my opinion they are doing no such thing. Your mother didn't dangle an incest carrot in front of you by giving birth to you, or by having breasts. And if you have a sister, she also doesn't exist to dangle a carrot in front of you. Neither does you sister dangle an incest carrot in front of you by not having died quickly, or by not having run away from home, or, gasp, even by going through puberty.
The incest taboo isn't just a taboo of all societies worldwide; there is a natural biological process that prevents people from being sexually interested in anyone they grew up with or anyone who grew up with them. It obviously doesn't work in all cases, and can be overridden in other cases. But everyone in Pale Carnations behaves in ways that are 100% compatible with the biological anti-incest mechanism being 100% working in their cases.
PS: You could argue that different to real life and regular fiction, in porn every living female relative is an instance of
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. However, Chekhov's gun need not be fired by the protagonist. It can instead be fired by someone else. Or the protagonist can rely on it only to discover, when he needs it, that he forgot to buy ammunition. Or someone else can use it to beat the protagonist to death. Or it may become the reason for the police to search the protagonist's house, in which search they find his stash of illegal drugs.
Returning to female relatives, in the absence of very specific signals, most of us will not jump to the conclusion that the protagonist will have sex with them, because most of us are actually not interested in incest. Though of course a lot of porn consumers are, making it economically rewarding to include this kink. But there are enough people without this kink to make stories without incest perfectly viable as well.