Pinkie Pie, as a member of the Pie family, actually has a twin sister: Marble Pie!!!

In the episode Hearthbreakers, Pinkie mentions that Marble is only a few minutes younger than she is. That would imply that Pinkie and Marble Pie are fraternal (but not identical twins, since Pinkie is a pink pony and Marble is a grey-coated pony) twin sisters.

Their mother Cloudy Quartz in the case of Pinkie's and Marble's births actual gave birth to fraternal twin girls.

Of course, after Pinkamena Diane Pie grew up, she left the family rock farm, moved to Ponyville, and obtained a job at Sugarcube Corner as the employee of Mr. and Mrs. Cake.

As for Marble Pie, will she develop a budding romance with Big MacIntosh?