Fukyagakusei no Himawari

Hiroo's brother, Nao, ran away from home at 18 because of disagreements with their parents. When Nao died in an accident, Hiroo decided to go to his house, where he had lived with his lover, to sort out Nao's belongings. There, for some reason, he found his brother still “alive” in the house. Apparently, Hiroo has traveled back into the past...?

(Source: Shueisha, translated)