You know, I never really thought of it like that, but... you're right. Satsuki IS a "mother figure" moreso than a child, and through most of the movie you see her trying to, I dunno, mat down her instincts and act as a surrogate for their mother. Which is why the few moments when you DO see Satsuki at her purest, most childish form are generally the most memorable.
I find it to be, probably, the most relatable aspect of the film, actually. Trying to act more grown-up is perhaps the biggest trend at that period of childhood, and given the circumstances, it's thrust upon Satsuki moreso than the average kid.
...
That was rather rambly, I think, but, yeah. Very insightful.