Kuro to Kin no Akanai Kagi.

Yori and his twin sister Iku used to be very close as children but lately Yori has grown colder toward Iku and is always eager to stay away from her. Iku was confused by Yori's attitude. Actually Yori is in love with Iku and that forbidden love is causing Yori sleepless nights since Yori and Iku are sharing the same room with bunk beds. Yori tried to ease his intense desire for Iku by bedding Iku's best friend. He had even applied to attend a boarding school in another city to take him far away from Iku. Yori lost his resolve to stay quiet about his feelings when Iku started to talk about having a boyfriend and going on dates. Now Yori want to have his little sister and he confessed. Iku loves her big brother and wanted them to stay together, so Iku is willing to anything to keep them together. But Yori wanted to have Iku as a lover, not as a sister loyal to her twin.