Lives inside wouldn't be the right way to word it. "She" always has been in there. Technically, and literally, she is Sora's memories. Thus, she's memories of Sora since he was born, from the raw power, made into form. The "live on" is not really inside anyone, as she doesn't have a being. It's just...Sora's memories returning where it should be, basically. And Roxas being Sora's Nobody, will carry his memories until they fuse. The more Sora lives his life, the more it will pack on. Unless the fragments suddenly become her silhouette because of the connection [which would mean he would have memory problems again], then "she's" no one in the end. Just energy. It's kind of odd to think now that "Xion" is what Naminé can manipulate.
Either way, the many Soras now is getting quite old >.>. He's the ultimate birth mom.
Then again, there's Xehanort and his many lives...