User blog comment:IamJakuhoRaikoben/Chapter 492: Elder Sister and Younger Sister, Review./@comment-4829046-20160704185220/@comment-4829046-20160705134831

Trust me, the black-white swords that also have humanoid forms thing is nothing new. Usually they are initially paired with protagonist and antagonist respectively but more often that not, they come under ownership of one person.

It isn't about them being bound to meet, it is just that Mashima sped it up by just making sure Sorano appeared right where Yukino was after the land shifted and making it so that neither had to wonder about the other's identity. Some actually brought up how Kagura recognizing Simon was also convenient, but there were a lot of reasons as to why that was the case. Here, it was just instantaneous without even getting closer to confirm.

Not sure about the Deathflags tbh. It might be a case where the flag merely switches over to Sorano. (before Mashima negates all tragedy at the end that is)