Well Memories seem to be a tricky thing. Even if they never met, it's possible that they affected Sora.
I mean, there's vast examples, and although each have their special circumstances, who are we to say there isn't another one on this case.
Point in case:
Riku Replica has memories of Kairi.
Sora has memories of Namine, before they meet.
Loads of potentials of Xion's memories, and hwo others' have affected her.
And the fact that people seem to lose Memories for all but coherent reasons.
I know my examples don't particularly make a strong point, but the point is, it's there.
Repliku has memories of Kairi because Namine
Sora has memories of Namine because the whole entire time he's been in Castle Oblivion, she's slowly messing with his memories until he forgets about everything except her
Don't know what you mean by Xion's memories. ;A;