The step-stuff is 90% just in the title, in the "plot" there isn't anything like that. Most people in porn aren't in it for the acting. The time of porn with a plot is pretty much dead.
From what I've heard it's zero-effort marketing: you make the content and rename it to whatever you want. The people who are into that will watch it and the rest of us will tolerate it.
Similar to foot fetish stuff, there's a surprising amount of people who are specifically into it and will see/buy anything with feet. The rest will kinda tolerate it.