I guess if Applebloom is the Setsuko and Applejack is Seita, then Big Mac is probably the father who was in the army/navy (?) and never came back and Granny Smith would be the mother who died after the bombing I like seeing crossovers like this, even though it's very sad, I can still imagine it working well, and it can bring MLP-fans and watchers of Grave of the Fireflies together
Ah, Grave of the Fireflies, a movie that exists solely because people like seeing terrible things happen to a little girl. Seriously, there's no plot, it's just 'This girl is dying because her brother is a douche. Feel sad'.
Actually, it was made for specific reasons, if you'd look more into the director. And the original was made because the guy who wrote it felt guilty that his sister survived the war while he didn't. :/ It wasn't meant to have a big complicated plot.