LOL, Christina never has been and never WILL be a soprano. That has to be the most laughable claim I always read about her voice. All of her power resides in her lower/middle register; that is also where she has the strongest intonation and the most color. Her voice noticeably thins out the higher it goes, and she loses a massive amount of resonance and overall vocal weight. The last note she can really even get away with in full chest is E5, which sounds like a blatant yell; above that is all mixed; and mostly head-driven, at that.
As for Christina's "whistle" notes; they are more like falsetto; the kind that you can reach just from screaming. The only difference is, she practiced hers and sort of "forged" it into an area of her range. But it's nothing special, and nothing that requires a high vocal range, as it is DISCONNECTED; you don't even need to be a female, much less a SOPRANO to reach those notes in that register. Just run a search on youtube for "Christina Aguilera whistle register vs youtube."