Neechan no Koibito

When Momoko Adachi was in high school, her parents died in an accident. She was left with her 3 younger siblings. To support her family, Momoko Adachi gave up on going to a university and picked up a job at a hardware store. 9 years later, she is now 27-years-old. Her 3 younger siblings are 20, 17, and 14-years-old. Momoko Adachi places her sibling's happiness before anything else. One day, she meets Yoshioka-san at work. She is attracted to Yoshioka-san, who smiles all the time, but he has a secret.
(Source: AsianWiki)