Hmmm.... well weren't they in the same grade? If so I don't think they can't be souers
(at leasts that's not what the souer system is about ^^)
Also, I got a question for kj1980. Does she got that much more screen time in the novels? If so, what would be her important contributions
(because she is circumstancial at best in the anime, being her only important appeareance we everyone was bugging Yumi about she rejecting Sachiko's rosary and Katsura tried to defend her...)