In my headcanon Dinky is actually four or five years older than any of the CMC. Her mom is Ditzy Doo AKA Derpy Hooves and Dr. Time Turner Hooves (Whooves) is her step-father. I’ll delve into their whole family ordeal later but the brace on Dinky’s leg is a clue.
Dinky was old enough to enlist when the war broke out and she became a paratrooper. Being a natural dare devil and overall brave pony, Dinky found no trouble diving headfirst out of a plane and into battle. She did so for four years until she returned to boot camp in order to train to become a full-fledged pilot. After the war was over she remained in the Equestrian Areoforce as a lieutenant.
Her cutie mark is one of the ones from the show and is for something, I’m not sure yet.