Wait... he sees a little girl that looks almost exactly like his older sister.... and.... he doesn't notice??? I mean, I get that it's all in her imagination.. but... still... I mean, it's semi implied that it might have been real at the end... that should count for something... but it's probably his imagination too... plus... she didn't recognize him despite always seeing his pictures...
I mean, again, I know it was probably just a dream... but... they should have recognized each other (or at least he should have thought she was nee-chan)...