Deisui Shite Osotta no wa Suki na Hito no Otouto deshita.

From Good Girls Scan BL: A marriage announcement from his first love! That night, after giving into his despair and getting drunk, Akira forced his first love’s younger brother to sleep with him. Through the years, Haruto started looking more and more like his older brother, so Akira tried to avoid him. He didn't want to develop any strange feelings towards Haruto, but it still ended up happening…! Akira is afraid that he traumatized Haruto, who genuinely tried to care for him. But then… ”Don’t act like that in front of anyone but me.” After just one night, he suddenly acts like a boyfriend!? An extremely cheerful Haruto starts chasing after him, and although surprised, Akira follows his lead. It wasn’t supposed to be serious at first. But Haruto becomes more and more important to Akira as he sees Haruto’s admirable devotion…