"Stuck between imouto and osananajimi..."

For our protagonist Souichi it is a daily routine to get raised by his Imouto Hazuki, and then go to school with her and his osananajimi Kotone.

Someday, his parents suddenly had to go on a business trip for half a year.
While Hazuki was made to go shopping, his parents told him that she is in fact not his real imouto -- that they don't have any connection by blood.

Hazuki was secretly peeking at this scene.

After she learned that there is no blood-connection between them, she approached her beloved brother.

Kotone noticed that, and in order to keep Souichi's desire away from his little sister, she had sex with him...

And thus his H-days began -- stuck between his proactive-perverted imouto, and his osananajimi, who tried to keep them from doing "it".

[Mostly translated from Getchu]