Questions (spoilers)...

Okay, so this is my take on what happened.

1) I'm pretty sure the baby was real. He was probably still in the crib or something in the house by himself when his parents finally died. If you think about it, it's almost ironic that (if the story continued) he would probably have to go up for adoption now, since his parents and grandparents are dead.

2) The grandmother died because she never liked Jin-seong and always talked about him badly, treated him poorly etc. Because Jin-seong was buried under the tree and kind of became one with it, it hated her just like he did and killed her when she smelled the flower.

The grandfather died because he knew about what the parents had done in killing and burying Jin-seong and helped them cover it up. He was also threatening the little girl, who was a friend of Jin-seong, probably because she knew what had happened. That was why she said something to the effect of "Why are you asking me when you already know what happened" when he asked if she knew something about Jin-seong's disappearance. She was afraid and went to the tree for defense, the tree cut the grandfather's cheek and he got mad and tore down a branch. That was a threat to the tree and, as he explained would happen before, the ants there would attack anything that tried to hurt the tree. Especially now that Jin-seong was part of it.

3) I'm not sure if the little girl died or not, but personally I think she did. She said she didn't have enough blood (which is why she wasn't in school), which leads me to believe that she had some sort of blood disease. She may have been intended to have Leukemia or at least some sort of anemia. I think anemia has something to do with not having enough blood. I got that from her appearance, she was a little paler than everyone else and always looked tired, kind of had circles under her eyes etc. Something like that can be pretty damaging on a body, and I would imagine especially a child, and maybe all the stress of her friend dying, witnessing it and then seeing what occurred with the parents and the grandfather, maybe it was too much for her.

It is not cleaar if the girl died or not ..i think not cause someone had to say to the policemen the secret about the tree.it was very strange that they knew where to search for the dead body without destrying the tree..And that they knew that the boy waas there when noone else was alive to speak..