Tsukumono Yume (つくものゆめ)/theories

Kozuki and Amatsuki are siblings
Kozuki and Amatsuki dress and look very similar to each other. Many areas, events and endings show that they are connected to each other, although how exactly is left unclear in the game.

One area in Amatsuki's dreams is what seems to be Kozuki's house where she lives with another woman (maybe their mother/some other relative). This would suggest that Amatsuki lived with Kozuki before finding his own house. Alternatively, Amatsuki had already moved out before/after Kozuki was born but still frequently visited her.