Our protagonist (Shunichi) is a college student with two younger sisters who attend different schools. Though they are old enough to start living on their own, they all commute to school from their parents' house because they feel it suits their needs. However, their parents died in an accident, leaving Shunichi as their older brother and a parent figure.

While he loves his two younger sisters, he gradually begins to develop desires for them.

He tries not to be overly concerned about the fact that they are siblings, but the plumpness of their newly developed breasts and their clear, white skin are enough to make him want them more than ever. Then he saw one of his younger sisters masturbating, perhaps through an act of force majeure...

The two sisters are aware of him as "the opposite sex", despite their different ways of expressing their affection, but what should he do as their "older brother"?

[translated from DLSite]