Man, this question is right up my alley. I think a lot of the appeal in these stories isn't just the 'step' part, it's the massive weight of the secret. That tension of living with someone you 'shouldn't' want, where every casual touch at the dinner table feels electric because you both know what you're thinking. The desire feels forbidden not just by society, but by the family unit itself, which adds a layer of guilt that can be delicious to read.
I lean towards stories where that guilt and fear of discovery are almost characters themselves. It's not just about the physical act; it's the whispered conversations in the basement, the coded texts, the absolute terror and thrill of almost getting caught by the other parent. That constant risk makes the payoff so much more intense. A story that glosses over that psychological mess and just jumps to the sex usually loses me.
I recently read one where the step-siblings were forced to share a room during a family reunion, and the build-up over a week of pretending to be asleep while listening to each other breathe was agonizingly good. The author really nailed the slow, painful erosion of their resistance.