I have a pet hypothesis that, on average, those into step porn are more likely not have grown up with a sibling of the gender they fancy. I reckon those of us who did are more likely to repelled by anything alluding to incest.
People who grew up with ACTUAL hot step siblings notwithstanding.
It’s a reference to many step porn scenes where the step sister gets “stuck” in a washing machine, or under a table, or bed. The step bro helps them out usually by fucking them out of where they are stuck.