From MangaHelpers:

Café San Francisco was named after the city Ichiro and his wife Machiko fell in love with when they went on their honeymoon. But when she suddenly passes away due to complications with her health, Ichiro’s five year old twin daughters volunteer to work at the store just like mom used to. Tsukushi is the reliable and mature one who looks like her mom while Mio is the clumsy and spacey half who gets her looks from her dad. The news of adorable five year old waitresses bring lots of business to the café including Hazuki, Machiko’s long time friend and friend of the family, and Miyajima, a college hopeful studying for his entrance exams. Together, they become a close group that spends their days at the café, enjoying each other’s company. [vyc]