A couple of good points worthy to ponder on. But before you continue reading the below list I would like to suggest to watch the movie first because it's a totally great one. In fact Miayazaki himself denied the following arguments and it might be a coincidence...
- It is saying that Totoro is in fact messenger of Death, and whoever sees him will soon die. The hospital that the sister's mother was in was based on a real hospital for terminally-ill patients.
- Later in the story the villagers find a slipper in a pond, which is in fact May's, at this point she has already drowned in the pond. Satsuki lied that the slipper wasn't Mei's out of denial. Ever since this scene, the sisters appeared to have no shadow.
- Satsuki pleaded the Totoro and the cat-bus to take her to where Mei is, while on the cat-bus, says "Nobody can see us...", this scene is Satsuki leading herself to the land of the dead (by taking the cat-bus).
- At the hospital, the mother says "I think I feel May and Satsuki smiling there in that tree..." Why don't the sisters go and see their mom if they are already there? Why do they just leave the corn there instead? It is said that the sisters were dead at that point, and the Japanese pronunciation of "corn" is similar to "kill child".
- The final scenes seem to be a happy epilogue, but they in fact happened "before" the major events in the movie.
- The movie was set in a place in Japan where there was a case of murdering of two sisters which happened in the 60s. This event took place on May 1st, while the sister's names are Satsuki (May in Japanese) and Mei (May in English). In the real life case, the younger sister was missing first and the older sister was seen to be looking for her frantically. Next day, the younger sister's body was found in the forest (stabbed to death). The older sister was in such a state of shock and kept rambling ambiguous words about seeing a "cat monster", "great big racoon monster" etc to the police. The sisters were in fact from a single-parent family (mother died of illness).