I don't necessarily agree with it completely, but Matt had a pretty interesting take on it. He said something along the lines of, many of the people watching these videos don't have jobs or leave their homes often so the idea of somebody living in their house fucking them seems more plausible and therefore makes it more engaging for them. It's the only way they can imagine themselves actually meeting somebody to have sex with. I don't think you can underestimate the taboo factor though. It's the old mentality of 'i know this is wrong but the attraction is so strong I can't help but do it.' I really don't think that many people want to have sex with with step parents or step siblings.