Actually, it's the other way around. Her being the Oblivion was the easier part to establish - hence why it was done first. However, with the Oblivion's link to Riku - it means Xion has a link to Riku, which could explain Snarl of Memories.
Also, there's the issue of wielding between them and proper Keyblades being owned or not.
And with them being together for so long but us getting to see all but none of it, well...
I really think there's something deeper and bigger there, and wouldn't be surprised if she ended up being related to Riku being able to wield again and having his own Keyblade.
I certainly hoped so, or that would be a lot of hard work down the drain.
At the very least I'll tell myself it's something on one else managed to phrase like I did :C