Koi Nanka Shitakunai - Kyou kara Kyoudai ni Nari

When Nakajima's mother decided to remarry he was afraid but everything turned out to be even better than he expected. His new older brother helps him and spoils him a lot and they're getting along very well until the moment when Nakajima comes home earlier and sees his big bro playing with another guy in bedroom. What's gonna happen from now on?