So what really happened in the end

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Yes, the wife really did revive. But then died again. Perhaps she needed immediate care/medicine? The girl in Red was the ghost of the child they aborted.
The part I do not understand is the photographers studio. Was that the afterworld? Was the boy dead? When he left the studio, did he return to the world of the living, or was he, like the girl, now a ghost?

< lots of SPOILERS >
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I read the story slightly differently to the other people here. Here's my interpretation.

The little girl in red is the aborted child of the husband and wife. The photographer's studio is a sort of 'netherworld' between life and death. When we see the father alone with the daughter at the beginning, it is during the period when he was 'dead' and recovering from a tumour. Now the husband is back in the real world, but the daughter is left, unwilling to continue through to the world of the dead without her parents.

The girl in red leads the son away from his policeman father, and hides him in the photographer's shop / 'netherworld' because she wants the policeman to visit her father and stop him reviving her mother: at least then she'll have one parent with her. In the end, she gains both parents, since the father is killed and, as a consequence, the mother sees no reason to fully revive, and allows herself to die, also. We see the three of them reunited at the photography shop, presumably on their way to eternity together.

The only thing which complicates this slightly is that the girl had not been aborted during the father's time 'dead', if I remember correctly. I think this is reflected in the fact that she is not photographed with him, and does not speak to the photographer (who comments on her silence). She is, in this first scene, a 'person in potential', in a sense: an echo of a future ghost, who only comes into true being when her mother falls ill, and hence by the time the policeman and his son move into the block of flats. (The fact that the girl is the only true ghost might also explain why she wanders around outside the photographer's shop, whilst the mother does not.)

The girl in the red dress is the unborn daughter Yu and his wife lost. The little boy...has always been a ghost from the outset of the story. He died prior to everything that takes place in the film. He can't move on because his father can't let go.

According to the film, letting go is beneficial to both the one that passes on and the living left behind. Otherwise both parties become trapped and can no longer pass on or move forward. This is evident at the end of the film, when the son is still lurking unnoticed in the photographers studio. He is a ghost who is sustained by his father's (the cop) inability to let go.

Alternately, there is a similar mythology built up around traditional medicine. Yu sustains his wife's spirit and keeps it rooted to her body while it heals.

I guess both Yu and the cop were doing the same thing, but in different ways. I'm disappointed that more of a parallel wasn't drawn between the characters and their circumstances while Yu had the cop confined.