She turned around, her hand resting on his chest, right over his racing heart. "They see the history, Selvam. But I see you. I’ve always only seen you."

Meera looked at her sister—young, naive, radiant. She thought of Arjun’s silence, the way he always asked about Kavya’s grades, the way he saved a seat for her at the temple. Meera had assumed those gestures were for her. But perhaps she was just the sister. The responsible one. The invisible one.

காதல் தோணி by எழில்மதி GS - Goodreads

As Selvam stepped behind her, his fingers brushed the nape of her neck. The playful banter that usually defined them vanished. A sudden, electric silence fell between them, louder than the festival drums outside. He saw her reflection in the polished brass plate nearby; her eyes were not looking at the necklace, but at him, with a vulnerability that shattered the "sister" facade he had built for years.

If you are a writer looking to capture this niche, here are three pillars to remember:

A classic setup where the heroine falls for her brother’s closest confidant. The tension arises from the brother’s overprotective nature and the secret romance that follows.