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

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

Spoilers ahead, for those who haven't seen this film

I remember watching this film with the commentary on (on the UK double disc DVD release), and the person providing the commentary (their name eludes me at the minute) talks about how the film originally had scenes which implied

the father and Su-mi were having an incestuous relationship.

In fact, I remember the scene being available on the bonus disc somewhere.

This might help explain the unreliable nature of the narration/story telling, as Su-mi might be using the apparition of her Su-yeon as a way of dealing with the death of her sister and mother, and (the above spoiler).

This all makes me think that huge chunks of the film happen inside Su-mi's head. And the kitchen scene takes place in her head, or is partly fictional.

At least, that's my interpretation of it.

The answer is rather simple. The figure under the sink is Su-Yeon - she is the entity haunting the house. Prior to the final scenes of the movie, when the REAL stepmother returns to the house, all the scenes featuring the woman are actually Su-Mi. If you notice almost every time the stepmother speaks with the father, he has a very strange reaction. IF you look at it from the perspective of Su-Mi delivering all those lines, and keeping in mind she is clearly insane, it makes sense why the father reacts the way that he does. Those words would sound like complete nonsense. In addition, the only person we see taking the 'crazy' pills are the stepmother, which is because the father is giving them to Su-Mi (usually after she says or does something strange). This also explains the dinner scene, as it was actually Su-Mi at the dinner with the stepmother's brother and wife, which is why everyone acted incredibly strange while she was talking during the meal. Recall that when Su-mi is informed that her uncle will be coming for dinner, she says she will not be joining dinner - in other words, the normal "su-mi" personality would not be in attendance; instead, the "stepmother" personality would be present. Now, it would be easy to chalk all the "supernatural" activity up to Su-Mi being crazy... however, we know that SOMETHING is in that house. When the woman has the seizure during dinner, she sees Su-yeon under the sink. We know that the dinner actually happened because of the father's phone conversations - so we know this woman was real and she actually saw something. Take this further, and when Su-mi is readmitted to the hospital, the REAL stepmother is in the house and encounters Su-yeon's ghost (you can see the green dress is the same as was being worn in the picture that fell and broke when the stepmother discovered the dresser in the final scene of the movie). So indeed, all the apparitions of the mother were in Su-mi's head, however the girl in the kitchen/under the sink and the ghost at the very end of the movie were real.