Kare wa Otoko to Shita Koto Nai.

Yukio is a normal salaryman who one day becomes relatives through marriage with his former classmate Hijiri. Now brother-in-laws in a sense, Yukio and Hijiri meet again and spend time with each other. But, when they kiss during a drunken moment, they realize they might have a deeper relationship than just friends and brother-in-laws.

(Source: Fantasista Inc.)