Grumpy and antisocial
You know what? It would be interesting if Pony Life Pinkie Pie actually had five siblings (Marble, Limestone, Maud, Octavio, and one more). This fifth sibling could be the Rainbow Dash of the group. The other siblings could be something like: Maud Pie = Twilight Sparkle, Octavio Pie = Rarity, Limestone Pie = Applejack, Marble Pie = Fluttershy (well, duh!). If Twilight, Rarity, Applejack, Fluttershy, and Rainbow had similar personality traits to Pinkie's siblings, that would explain why she is friends with them… Well, it would be somewhat similar to what happened in Friendship Is Magic (more or less, I unfortunately don't have time now to be precise), but this is another timeline!
(I added the spoiler filter because I know there are folks following the Discovery Family airings)