I have to disagree on this one. I just saw this summer's Celine ad with Carmen Kass in a mag, and I think she's more suited to the brand than a girl as young as Sigrid. I see Celine as a brand for women like Phoebe Philo : women in their 30es+, confident, with life experience. A model like Carmen embodies that well, plus she has a natural elegance (at least in the ad).
That said, Sigrid has a natural elegance too, her blond & nordic look fits Celine too, but she is a tad too young to be a perfect match IMO.