User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-139.195.206.252-20130329101437/@comment-7418318-20130329105805

There are more hints than just that. Like their similar looks, similar magic, similar dressing style and the parts of Sorano's (that's the name of Yukino's missing elder sister) face seen in a flashback fitting very well to that of Angel, including the hair ribbon.

I also read that in the japanese manga both Sorano and Angel use a very peculiar "dialect" (though that wasn't taken over into the Angels scenes in the Anime). Not having seen the original manga myself though, I can't be sure about this bit.