Eroitto - Ikenai Kozukuri 4 ~Namaiki na Aneki ga Jitsu wa Ore no Semen ga Daisuki Datta!?~

The protagonist is currently living together with his older sister and going to prep school everyday. He likes her but is unable to be honest about his feelings.
One day, he comes home early and see his sister masturbating while sniffing his shorts. At the height of his arousal, he unintentionally has sex with her.

After doing the deed and being questioned if she likes him, his sister denies it on the ground that she only likes his scent.

While disappointed at her words, he is delighted to know that from now on, he can make her do anything using his smell.