Limestone Pie and Marble Pie

Limestone Pie and Marble Pie are female Earth ponies and members of the Pie family. They are Pinkie Pie's oldest and youngest sisters respectively. They first appear as fillies in a flashback in The Cutie Mark Chronicles and appear as fully grown mares in present day in the chapter book Pinkie Pie and the Rockin' Ponypalooza Party! and season five episode Hearthbreakers.

Development
According to author G.M. Berrow, she named Pinkie's sisters Marble and Limestone so that they and Pinkie would have the initials "MLP", for "My Little Pony".

When asked on Twitter if Pinkie and Marble are twins, Hearthbreakers writer Nick Confalone replied "it comes down to your definition of twins for non-humans" and "Marble/Pinkie not identical twins, if they were I'd call them twins." Characters who have been referred to as twins include Pound Cake and Pumpkin Cake, Flim and Flam, Cheerilee and Cherry Blossom, Tofu and Tempeh, Antonio and Francesco, and Spa ponies.

Depiction in the series
Limestone and Marble first appear as fillies in Pinkie Pie's flashback in The Cutie Mark Chronicles tending the Pie family rock farm. Unlike their parents Igneous Rock Pie and Cloudy Quartz, Limestone and Marble have no dialogue in the episode. When Pinkie Pie throws her first party in order to spread cheer, Limestone and Marble are shocked at first, but they slowly smile and join in the festivities.

In season four, Limestone and Marble appear in a picture with the rest of the Pie family in Pinkie Pride.

In season five, Limestone and Marble appear as grown mares in Hearthbreakers when the Apple family and Pie family spend Hearth's Warming Eve together. Here, Limestone is depicted as very aggressive, acting as the primary caretaker of the rock farm. She is especially protective of Holder's Boulder, a large sacred boulder on the farm. Marble, on the other hand, is depicted as very shy and soft-spoken, often letting Pinkie speak on her behalf. She doesn't say anything other than "Mm-hmm", a characteristic similar to the equally laconic Big McIntosh, with whom she forms a friendship.

Marble and Limestone are mentioned collectively by Pinkie Pie on The Gift of the Maud Pie, explaining to Rarity how Pinkie meets with each of her sisters in various locations around Equestria for gift swap meets.

IDW comics
Of, Limestone and Marble appear as fillies on the cover RI, and Marble appears with Pinkie Pie on the Jetpack Comics cover and the Jetpack Comics/Larry's Comics shared cover.

In, Limestone, Marble, and their parents appear in present day on page 3.

Of, Marble appears in present day with Big Mac on the Jetpack Comics cover, which will only be available at PonyCon in New York.

Chapter books
In Pinkie Pie and the Rockin' Ponypalooza Party!, Limestone and Marble visit Pinkie Pie in Ponyville. They and their parents appeal to Twilight Sparkle for help in saving their rock farm, which they are in danger of losing due to rivaling businesses. As part of her plan to save the rock farm, Pinkie Pie suggests a big rock concert, and she puts Limestone and Marble in charge of decorations. Though the two have minor dialogue, they are described as having an "unspoken language that consisted mostly of blinks and nods."

My Little Pony (mobile game)
Limestone and Marble are available characters in Gameloft's mobile game, added to the iOS version of the game on June 6, 2014 and to the Android version of the game on June 13, 2014. Limestone's in-game description states "When Limestone Pie is spotted in a crowd, you know she's Pinkie's foal sister when things get super loud!" Marble's in-game description states "Marble Pie is a foal who doesn't take her family's rock farm for "granite"! She loves to play hide-and-seek among the boulders... That is, until no pony can find her!"

Quotes
"Gaze into the eyes of Limestone Pie. Ma and Pa may own this rock farm, but I keep it running."

- Limestone Pie, Hearthbreakers

"Mm-hmm."

- Marble Pie, Hearthbreakers

"Stay off Holder's Boulder!"

- Limestone Pie, Hearthbreakers

"What. Happened. To. My. Farm?!"

- Limestone Pie, Hearthbreakers

"Well, don't just stand there! We got a boulder to move! And I'm in charge."

- Limestone Pie, Hearthbreakers

"It's the gems!"

- Limestone Pie, Pinkie Pie and the Rockin' Ponypalooza Party!

"Well, you tell that to the rest of Equestria. We haven't had any business for months!"

- Marble Pie, Pinkie Pie and the Rockin' Ponypalooza Party!