plot explanation - What was under the sink in A Tale of Two Sisters?

In A Tale of Two Sisters, during a seizure, a woman happens to see something (someone?) under the sink, and she tells her husband about it.

We later see a hand come out from under the sink, but due to the nature of the movie (a very unreliable narrator), we can't easily know for sure if the hand was real. However, we have no reason that I know of to distrust what the first woman saw. Was there a tie in with the rest of the story somehow? Or was this just a red herring?

Answer

I saw this movie recently and from what I could gather, everything that happened (except the part where the older sister returns to the house after her treatment and the bird being killed) is imagined...it's in the mind, because of her dissociative identity disorder. The other woman couldnt have arrived at all, I think, because all the while the stepmother was not present at all in the house. She arrives only much later. So if the stepmother wasn't there, and it was the girl imagining everything, even these incidents are part of the supposed imagination. A bit far fetched I know but this is the best I could comprehend.

EDIT: It appears my interpretation of this was skewed. The family dinner did happen and the woman did see something. It seems that what she was is supposed to be a ghost of the dead sister. Please see this link for a detailed discussion on the movie:

http://www.goodreads.com/topic/show/135340-a-tale-of-two-sisters