In Pinkie Apple Pie, it is said that Auntie Applesauce is the fourth cousin twice removed of a member of the Pie family. But who is it? I believe I can answer that question definitively, with science (er, more with logic and canon, but still)!
Firstly, if Applesauce is their 4th cousin twice removed, that means Applesauce is two generations above them on the family tree (a fourth cousin to their grandparent). This means they'd be around the same age as AJ's parents (if AJ's parents were still alive, that is). This gives two rather obvious candidates: Pinkie's parents. Now this leaves two possibilities: either Igneous Rock Pie's mother was an Apple, or Cloudy Quartz was an Apple who married into the Pies. Given Cloudy's name, I find it unlikely that she was an Apple, leaving Iggy's mother: Granny Pie.
Granny Pie was originally an Apple who married Feldspar Granite Pie.