Update: Steven Greydanus offers a rebuttal to this theory. An excerpt:
My spider-sense says this is like those economic or political crypto-readings of The Wizard of Oz that marshal a lot of seemingly significant coincident details in the service of an over-developed conspiracy theory that is hard to refute but which completely misrepresents the real genesis of the story.
Some perspective: The story’s origins are semi-autobiographical. Miyazaki’s mother was hospitalized with tuberculosis when he was a boy, and his father brought him to the country. His mother was hospitalized for years but eventually left the hospital and lived at home with the disease for years. She did not die in the hospital.
For the film Miyazaki decided to make the protagonist a girl rather than a boy, and later split the girl into two girls (with essentially synonymous names of Mei and Satsuke, both of which mean “May”). The idea that they were inspired by the two girls in the murder case is false.
At least, that’s the theory put forth by one fellow. Some evidence:
The real story comes from the history of the Sayama incident (狭山事件 sayama jiken). There seem to be too many coincidences between the Sayama incident and this movie to ignore.
The Sayama incident occured in May 1963. It’s quite an important case for discrimination in Japan. The case goes that one day, in Sayama (in Saitama prefecture), a young girl was kidnapped for ransom, raped and then murdered. Her older sister apparently found her body, but was so traumatized by it, when asked what she had seen, she merely said “I met a large Tanuki (looks like a racoon)” and “I saw a cat monster.” Sound familiar? Anyway, the older sister later commited suicide.
The Nekobus (the cat bus) is the cariage that takes one to the next world (heaven, hell, whatever). This is given a little reinforcement by the above picture, showing the destination as 墓道, the first character means grave, the second meaning road.
So in the story, the idea is that Mei is murdered after she goes missing. Satsuki, feeling greif decides to join her.
In the scene where Mei is lost and crying, she is sitting next to 6 地蔵 jizou Jizou statues (English Ksitigarbha, a buddhist deity that looks after the souls of deceased children and aborted fetuses in Japanese culture. Apparently these 6 Jizou statues represent 6 people who died in the course of the incident (see above).
Apparently one of the phrases in the lyrics of the song of the Nekobus is “乗ったお客は陽気なおばけ” notta okyaku wa youki na obake, “those guests who ride are cheerful ghosts.”
Studio Ghibli has denied this, but honestly, I wouldn’t at all be surprised if some of this were true. Ghibli films often have darker, more complex themes and undertones going on. Which is one of the reasons why they’re so great.