Onee Shikkaku: Kedamono ni Hyouhen shita Gozen 3-ji

Emi Asai, a run-of-the-mill office worker, is currently living an apartment with her older cousin, Kiichi Uno. A good-looking guy, Kiichi works nights as a bartender. Best friends since forever, they couldn't have been more different in their daily schedules, or their choice of lovers... Until one late night, when their relationship gets turned upside down!

(Source: Renta!)