The main character lives more or less a secluded life working as an ero mangaka... Suddently his younger sister Sora arrives at his home, telling him that their father has left a goodbye note and is nowhere to be found.... Their mother is long gone, so being the only relative that can take responsibility for her our hero decides to take care of Sora and provide her with shelter and a shoulder to rely on.
Early on, the main character investigates a little, and realizes that he and Sora have no blood relationship at all... He decides to kept this little tidbit to himself sans Sora would be truly be left without any family left. Of course, in order to complete the set up, we have that Sora has a massive brother complex, and that she gets obviously jealous whenever her brother seems to get a good mood with his long time love interest.
We have that they will need to survive some big hurdles that will pop up on the long road in order to stay a family, with way more problems that they could ever foresee. Will their "brother and sister" relationship truly last forever?