Imojoku

Our protag doesn't get along well with his little sister. She hates him so much but he's fine with it. He lives a simple and happy life... until his girlfriend dumps him without giving any reason. His (ex-)girlfriend won't come near to him nor even talk to him and he is clueless of what exactly the reason why. The only way to know the truth is through investigating the cause of the breakup-- his very own younger sister.

Investigation is useless as she adamantly refuse to spill the beans while arrogantly claiming to be the culprit. Then he found a way to make her sister fully spill the beans----by raping her until she speaks the truth!

[Edited from this original description by soketsu]