Honestly, I've been trying to wrap my head around this one for a while.
So firstly, I think it just has a lot to do with the memetic way porn is marketed. Certain key words become popular and get repeated. It doesn't really matter what the content is, you put popular phrases in the title anyway so that it shows up in searches.
But in terms of the actual fantasies, I suspect it's probably to do with men's typical experiences of adolescent sexuality. Step sister/other same age fantasies primarily seem to be about recreating conditions of sexual exploration between teenagers. They often incorporate things like the risk of getting caught, concerns about premature ejaculation and so forth. There's often some element of upward mobility, with the "sister" usually being implied to be more popular or sexually experienced. As for why it's an incest thing, I suspect it's less to do with the taboo itself for most people and more to do with the idea of a safe, loving environment in which sex can nonetheless be spontaneous and unexpected.
Step mom fantasies are a bit more obvious. They recreate adolescent fantasies about older women, but with the added element of an established caring relationship.
Step daughter fantasies are the only ones that I think are very clearly about the violation of a taboo, because the viewer is projecting into the position of the supposedly mature adult, but even then I think the taboo is often less incest than it is adultery or an implied age gap. Again, I think the quasi-incest element far more about rationalizing why these characters already have a close and loving relationship and yet haven't had sex already.