Writing a step-sis romance story requires balancing tension and taboo while keeping it emotionally grounded. First, focus on the 'forbidden' aspect—the societal pressure, the family dynamics, and the internal conflict. I’d start by building a backstory where the characters weren’t raised together, maybe meeting as teens or adults after their parents marry. That way, the attraction feels less icky and more like two people caught in a messy situation.
Next, pacing is key. Slow burns work best here—lingering glances, accidental touches, moments where they almost cross the line. Throw in external obstacles, like a disapproving parent or a nosy neighbor, to add stakes. And please, avoid making it purely about the taboo. Give them real chemistry—shared interests, inside jokes, vulnerabilities. Think 'The Idea of You' but with way more complications.