Fukyagakusei no Himawari

I am Hiroo. My brother, Nao, who had left home at 18, died in an accident. Nao ran away from home because he had a bad relationship with our parents. Hiroo decides to go to the house where Nao used to live with his lover to sort out his brother's mementos. Then, for some reason, he found his brother still "alive" there. Apparently, Hiroo flew to the past...?

(Source: Shueisha, translated)