Imouto

Yoshugara Hikaru and his little sister, Mioto, are childhood friends of Sugihara Hakuro, and Hakuro's little sister, Akimi. Although Mioto and Hakuro love each other, Hikaru is also secretly in love with Mioto. One day, Akimi walks into Hikaru’s room whilst he was playing with himself while spying on Mioto and Hakuro. Akimi demands that Hikaru finds a way to break-up their sibling as Akimi also loves her brother. Hikaru refuses her but Akimi threatens to reveal his shameful act. Hikaru loses his temper and rapes her. Horrified by what he had done, Hikaru eventually agrees to cooperate with Akimi on condition that his indiscretions be kept secret.
(Source: AnimeNfo)