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So Nana Pinkie. My first thought was did they retcon Granny Pie? My second thought was everypony has two grandmother's, duh. So Granny Pie taught her how to sing and face her fears and Nana Pinkie taught her all kinds of obscure stories and legends that happen to be true. Nana Pinkie is probably how Pinkie Pie knew about the parasprites as well. Since Nana Pinkie is probably two for two if Twilight learns about her she'll probably want Pinkie Pie to tell her every story so they can be written down for future use.