User blog comment:RennaGraceCarter/Yukino's Sister/@comment-5936871-20130418134349

I had the EXACT same thoughts. I also found it strange how they showed her sister's lower half but they didn't show her face. Which makes me think she's someone we already met before during the series. Yukino and Angel look similar, and they're both Celestial Spirit Wizards. And considering her sister was captured as a child by Zeref's followers (who at the time were enslaving children to make them build he Tower of Heaven) makes pretty likely IMO.