we've had hundreds of threads over this, and the general response seems to be almost nobody likes the real incest (as in nobody wants to fuck their OWN mom nor daughter) but only the fictional version. reasons vary a little, but my understanding is the core of it is experiencing the forbiddenness of the idea, WITHOUT involving your real family which would make you feel sick about it. if the daz model was replaced by a scan of your real mom that would be a deal-breaker for most.
then there's also more practical reasons like mom/son games being an easy access to milf fantasies, and father/daughter games to corruption of innocence stories. or shota games for re-experiencing the time of your life when you were a kid yourself and started drooling over some teacher, only this time the story is 'what if' she had accepted your horny thoughts. and I'm sure there's a whole bunch of other subgroups with their own approach to incest stories.
I have seen some rare people saying they'd actually want to fuck their mom/sister etc, but that seems to be incredibly rare within this genre audience. like way less than 1%. so rare that it doesn't really even make sense as the abusive real world incest isn't that rare. so I'm kinda thinking the real world incest people might not be getting what they seek from fictional incest, as it's not their family, so they might not be here to begin with.