Imouto Chu☆ #01 Sunao Imouto no Shou

The protagonist Onii Tomoyasu is a fairly ordinary college student. However, because of his name he has received a certain nickname since long ago : "Onii-chan". Thanks to that he has been called that way every day by both people younger and older than him ever since elementary school. That's how he's been called "Onii-chan" for 22 years despite having no actual younger sister.

And one day, when he turned around after hearing "Onii-chan" once again, he found a sole girl looking at him. The girl (Sakuranomori Fumitsuki) told him she is his half-sister. While our protagonist is still surprised at the sudden appearance of a sister, she continues her story and asks him to come over to their house. She came to convince him to help her sisters. She tells him their father's behavior caused them to become distrustful of males. And when he asked her what he should do in particular, she answered... "Please have sex with them".

[Translated from Getchu]