No, you are thinking of Dinky Doo, the Unicorn foal that mysteriously popped out of Derpy's oven after her brother,
Dorky Doo DR. INSANO DOOFENSMIRCH!!!
, modified it in one of his experiments with Hyper-time.
Since she apparently shares her mother's mitochondrial DNA, Insano thinks she some kind of recombinant created from a stray hair that landed in the Muffin Mix...
Here's a hint, she's not.
So far for Derpy's sisters, I have from oldest to youngest:
Air General Durhura, the eye-patch wearing eldest, a strict military badass based on Olivier Mira Armstrong
Twin Ditzy, the brilliant but clumsy mare who's struggling to find her place in the world, contrast to her sister who is not-very-bright but the scarily competent Ponyville Postmaster.
Youngest Zippity, the reckless speedster who got into the Wonderbolts due to her oldest sister pulling some strings on the claim that it was, "For the safety and security of Equestria."...Can't believe people wonder why I'm so odd...