Hidamari no Asphalt: Nee-san, Hajimemashita

Akari Usui is a girl with a rather unfortunate life, just as her name suggests. Having lost both her parents, she works at an accounting firm while burdened with debt. Meanwhile, Sumire, the young boss of the Warudakumi gang, is told by his critically ill boss that he will let him inherit the leadership if he marries.

He sends a female acquaintance, Choko, to pretend to be his wife, but by mistake, Akari is brought instead.

The lives of these two were never meant to intersect, but their paths crossed when they both found each other a little bit "cool!" and "cute !" Sumire decides to marry Akari, treating her as his "big sister"...!

A lonely girl and a young yakuza boss. Their worlds were never meant to meet...or so they thought!? A mistaken identity leads to intersecting destinies, a substitute marriage, and first love... A heart-pounding love story about a girl unaccustomed to happiness who is loved to the core of her heart!

(Source: Shodensha, translated)