Originally Posted by Hawkingbird
I thought he had a twin sister....
Yes, but the movie adapted that into a twin brother.
In the comics, Xavier's mind was transferred from his dying body into a clone created by the Shi'ar, and X3 combined that idea with the twin sister.
Of course, what sounds perfectly okay in the comics has an air of absurdity when in the films.
The twin brother mind-switch thing in X3 does sound a bit daft when you say it out loud.