Ore no Katachi wo Oboete yo, Onii-chan?

Sousuke, who is in his first year as a working adult, has a spoilt stepbrother who gets lonely easily. He spoils his younger brother, Mahiro, so much that he gets sucked into a relationship where they start kissing and having sex together. Sousuke knows that what they're doing is wrong but he can't help but give in every time the cute Mahiro seeks him and they continue this way until one day, Mahiro suddenly stopped having sex with him...