"Oniichan, please come home soon. If you don't..."

Takumi (you, protagonist) and his younger sister Anna lost their parents in a car crash.
When they moved in with their uncle Hiroshi, Takumi instantly disliked him.
For some reason Hiroshi took a liking to Anna...
Spurred by his uncle's advances, Takumi realized that Anna was the only woman he ever wanted.
He kissed his sister, and she, in turn...

Takumi knew they couldn't go too far, as brother and sister.
But that pervert Hiroshi... he had no problem crossing the line.

[From DLSite English]