...and I just read someone commenting that the Charles and Raven backstory means Charles in the X1-3 films acts like a hateful jerk towards Mystique by not acknowledging her. Forget Doylist, let's stay Watsonian - is he ever in the same scene with her? I think not. So yes, the foster siblings are a retcon, but it totally works precisely because we've seen no interaction betweeen Charles and Raven in the later/earlier films to contradict it.