From a lifestyle perspective, families must have honest conversations. If you are an OF creator using the "step-sister" tag, do your real-life step-siblings know? Do you live in the same house? Most ethical guidelines—and the terms of service for major platforms—require that be involved. The keyword explicitly says "for an OF video," implying production, not reality.
From a pure entertainment standpoint, the "big step sister" trope works for three psychological reasons: Fucking My Big Ass Step Sister For An OF Video
"You're welcome. Now pay me in tacos."