Totoro is the God of Death, so the persons that can see Totoro are actually close to death, or already dead.
This means,when Mei goes missing and a sandal is found in the pond, Mei actually drowned. When Satsuki is asked about the sandal she cannot face the truth and lies about it not being Mei’s sandal.
So Satsuki goes on a desperate search for Totoro, calling for him and actually opens up the door the realm of the dead herself. With Totoro’s help she finds her dead sister and they together go to their mother’s hospital. There, the only one who actually noticed that the sisters were there, was the mother, who also soon is going to die.
SPOILERS
If you look closely at Mei in the later half of the film, her shadow is more transparent than it was in the first half. As it is believed, the dead don't cast any shadows.