User blog comment:RennaGraceCarter/Yukino's Sister/@comment-7418318-20130418070343

I don't know about a dedicated blog for the matter, but the theory has been discussed in other blogs, like chapter reviews.

In one of the chapters you see a flashback that shows part of Sorano's face. Those features that are seen match up very well with Angel.

In addition to that and what you already mentioned, Sorano and Angel also seem to have a certain accent in common (at least in the japanese manga... the translations seem to have omitted it and I'm not sure about the anime voicing).