help me find this japanese movie

About a guy living alone in the city, he have a rich girlfriend. Her parents forbid their relationship because the guy is poor.

One the the main guy's little sister started to live with him. It seems that the sister was in love with the main guy (they're not blood related)

A months had passed the sister decided to move out for some reason. Then the main guy broke out with her gf.

iirc the ending was tragedy