User blog comment:FairyRaven/Fairy Tail Chapter 303: A Battle On Two Fronts, Review and Commentary/@comment-112.205.76.218-20121012105856/@comment-4933386-20121012114821

Now we have to think who might possibly be Yukino's sister, either will never be revealed or someone who will be working with Zeref (HE BRAINWASHED HER) or someone who hate mass hatred for Zeref instead.

haha every female in the anime.

I love how it doesn't reveal a face or hair as if we know that person, so I'm guessing it's gonna be someone we've seen before I reckon.