Zap2it just posted the episode synopsis for S4E18, entitled "Maud Pie". Get it after the break!
"When Pinkie Pie's sister comes to town, the girls find her much different than Pinkie described."
If the chapter books are to be considered canon, then taking into account that Pinkie's two younger sisters (pictured below) were given names in G.M. Berrow's Pinkie Pie and the Rockin' Ponypalooza Party! it seems reasonable to assume that the sister visiting Pinkie in the upcoming episode is the third older sibling first introduced by the aforementioned book who later appeared in the family photo from "Pinkie Pride" as seen in the header image above.
|So Limestone, Marble, and then there's Maud.|