I see Caria more as a source of information, not as an assassin. But, we havent really seen a lot of her or gotten a real handle on what type of person she is, so I could be wrong.
Another little detail is that Sextus confided in Miles (before his collapse) in AF that he had treated Caria porly and felt very regretful about that. Couple that with the prolonged effect that Max's little "improvisation" with Caria (when he poured on the Earthcraft at her) and I think that Caria may not be quite as receptive to helping Aquitaine as she was at the end of FoC. (Imaine that, Antillar Maximus saving the First Lord's marriage)
Having given the above opinion, a question I have not seen posed yet (but that I think does require some attention) is this: How does Caria react when Octavian is revealed and she gets pushed off to the side?
After all, when she was married to Sextus the plan (at least in the eyes of the other high lords) was for her to bear the child who would be the next First Lord. I cannot imagine that she would take Octavian's apearance and elevation to the title of Princeps very well...