Theoria is more of a title than a name (and more of a verb than a title).
http://en.wikipedia.org/wiki/Theoria
Wikipedia should help, but the gist is that Kano is referring to Ran's role as seer; as to why he insists she's his sister, it may be more of a symbolic gesture. He's broken into the realm of consciousness she's always inhabited, they are the only ones able to access that state, so he sees her as his "sister" in the sense that any group of people united under anything can be "brothers" and "sisters."
Anyway, I love Texhnolyze. It's a shame there isn't a whole lot of animation anywhere that matches it in depth and style.