I think that that is quite possible but maybe Xion has no relation to Kairi at all. If you follow the rule that Organisation 13's members' names are their somebody's name anagrammed with an X. It would mean that Xion's name would be Ino. What I think is that since Kairi is the "Seventh Princess", there may be other "Princesses" that might be able to use the Keyblades and they would be similar to Kairi in some way. This would explain why Xion has a Keyblade.
Another theory I have is that maybe Xion is not a Nobody, but perhaps someone pretending to be a Nobody, since Xemnas and other Organisation members refer to the Organisation as Organisation 13, not 14. It is probably because they found out that Xion is not a Nobody and they find out about it Kingdom Hearts 358/2?
Namine has special powers because she is the Nobody of Kairi. Kairi has the ability to affect Sora's heart. She plunged Sora's heart back into him after he was turned into a Heartless. What I think is that Namine also has these powers but that they might be a little different from Kairi's. This explains why Namine is able to mess with Sora's memories. I don't think that Namine can affect anyone's memories other than Sora's. In Chain of Memories, Traverse Town, Aerith said that Sora's memories are affecting theirs. I think that while they are in Castle Oblivion, Sora's memories can affect memories of those connected to Sora but only to a certain extent. This would explain why Donald and Goofy and Kairi were affected and forgot but could still know what they forgot, unlike Sora who didn't realise that he had forgotton about Kairi totally. Donald and Goofy only forgot King Mickey's name but not the warm feeling that they have when they are around him. Kairi forgets about Sora but she knows that she also has another childhood friend other than Riku.
These theories may be correct or they may be waaaay off but I think that they are probable and would explain how everything fits together.
