Step does nothing for me. I think maybe in the same way that the taboo nature of incest is what makes the idea hot in porn, step is like a bucket of icewater that only negates that but more than cancels it out.
If the game is not about incest then a fuckable step character is fine. Partially it's intent and implementation. of which of course an audience cannot be a perfect judge.
There is a sort of general principle that I'm having a little trouble clearly explaining. Impositions that try to make the fetish more palatable more than spoil it.
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There is another element of your response that is interesting.
"I would say the presence of a mother to sneak around/pretend to be a family with is more important."
I don't care about this at all. Like not even slightest tiniest bit.
My point is not to say you have misidentified anything or that your preference is an invalid, but that underneath the things we like are microfetishes and points of overlap. Two people can like the same fetish for very different reasons, which is interesting. (and a number of times I've come across something that crossed wires in a way that taught me why I like certain other things)