Mabey they made a mistake? They do that sometimes. Also, I have a theory that Maud isn't really Pinkie's sister. And I have evidence. (Le gasp! I got off my lazy butt and did some reaserch on this. XD) In the episode, The Cutie Mark Chronicles, none of her sisters were gray and had a dark purple mane, which means that none of them could have been a sister. This lead me to think that Maud is LIKE a sister to Pinkie, since in the Maud Pie episode thingy, she also kept saying they were best friends. And sometimes, people call their best friends, sisters.