I just realized that Geralt doesn't have his silver sword. His gestures look like he might be having it in hand, covered by the horses, but then again he doesn't have the empty sheath on his back either.
This allows the conclusion that the person on the horse behind is carrying Geralt's silver sword and need not be a Witcher after all. Or he/she is a Witcher and has it because he/she has lost his/her own silver sword. Anyways, I'm not quite as sure it's Ciri anymore - it could also be someone with a grey beard or bright long hair. There's nothing distinctively womanly about that person, but then again, the thick cloak and cowl don't show any features and Ciri doesn't behave very girly anyways.
For the sake of CDPR making a mystery of that person and because it fits very well with the story, I'll stick to the opinion that it most likely IS Ciri, but I wouldn't be too surprised if it isn't and it was a false clue, given to us on purpose.