Ai to ka Koi to ka, Kudaranai.

Kawai Yuuma had a childhood friend, Kuramoto Suzuka. She was a year younger than him, and also the little sister of his best friend, Akira. To Yuuma, she'd always been like a little sister, even as they were now in high school. That's what he thought, anyway... until one day, Akira fell for a senior at his part-time job. "That's insane," Suzuka scoffed, admitting she had no clue about romantic feelings. Yuuma himself had some baggage when it came to love, but he was curious about "that kind of thing." "I hear kissing feels good?" He knew it was wrong, but he couldn't resist crossing the line. They made a deal: this messy situation would only last until they found someone they truly loved. (Source: Gagaga Bunko, translated)