During the mid-1970s, Hayao Miyazaki began coming up with concept art for what would become My Neighbor Totoro.
Below is the scene that appears in My Neighbor Totoro, with both sisters standing next to the furry giant.
In an interview (via Spoon & Tamago), Studio Ghibli producer Toshio Suzuki explained that My Neighbor Totoro was originally going to be a children’s picture book.
The girl does look like a combination of Satsuki and Mei, which might explain who the girl is.