User blog comment:RennaGraceCarter/Yukino's Sister/@comment-49.146.205.254-20140504070318/@comment-7418318-20140504092557

As B214 said.

Yukino called her sister Sorano and a in chapter 365 Jellal called Angel by that same name.

But even before most of us were sure they were sisters anyway, so that was more like a "Cool, now we have official confirmation" rather than a "Wow, I never expected that".