OK, so Erin and Xavier knew each other from the refugee exodus to Q'Barra. I'm sure they've helped each other along the way and remained in touch in Q'Barra. After Erin learned of her excoriation (am I still going with that Urko? You had mentioned the possibility of not using that part of the story, though I don't think I got a definitive "no" on it), she decided, after a small period of self-loathing, to work some jobs alongside her old Cyran allies. This could also include Xavier, possibly along some of his escort jobs. If Urko approves, I like the idea that Erin had a "base" (such that it is) that she would have used for her craft, but it is now abandond when her other Cyran refugee friends left. Yeoman, what do you think of this: these people were also Xavier's friends, and the reason for the other refugees leaving Cyre could tie in to the reason for Xavier's failed suicide attempt. One of the reasons Erin stayed might be to look after Xavier. It would fit into her personality history: she was very cold and aloof before Cyre's destruction, but the shock of the mourning has thrown her the other way so that now, out of guilt from before, she has developed an attachment to her former Cyran allies.
For Sanae, I am still thinking about an event that would have brought her and Erin together, but I think that the dynamic makes a great deal of sense. Erin is emotionally unstable, and would be searching for a role model from an emotional stability POV, something that Sanae would appreciate. Sanae might see in Erin a way for redemption for the feelings of vengeance she felt as a child. The two have in common their being touched by higher powers: Sanae by the Silver Flame and Erin by the Dragonic Prophecy. Also, Sanae would be albe to sense Erin has a desire to do good, and wants to guide her down the right path... a path that might eventually change her alignment to "good" and bring her into the fold of the Church of the Silver Flame.
Sorry if I may seem presumptuous to work on other characters' histories, but this is the part I really like about character creation: fleshing out the personalities, motivations, and history of the party (and it's why I jumped at the chance of joining this PbP)