Imouto Moe

This is a visual novel that tells the story of a girl Moe who lives together with her brother.

The protagonist's parents are gone on a trip, so now he's living together with his sister. Up until recently, his sister, who is almost the same age, had been living elsewhere.
The siblings are at an age where they start to become interested in sex, and now that they are together they become sexually aware of each other. Then the brother gives in to his desire and assaults his sister.

[From DLsite English]