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