It's easy marketing. Convenient situation, taboo, but not too taboo. The line always seems to be "but you're my stepsister" "yeah, STEPsister, not real". So it's wrong, but not that wrong. The multiple people scenario is just ramping the taboo up, but not worrying about relationships after the video. Real polygamy is complicated and probably not as successful as porn depicts it. (and that's ignoring the step situation altogether)

I say probably only because I'm sure there are some out there that work. But to have three or more people in a long-term working relationship without costs has to be rare. Just having something like a work relationship between a few people can be touchy, and that's without feelings and sex involved.