The Tagline of the movie is The Family Just Got Bigger
Stuart Little: So, what do I call you?
Mrs. Little: Mom.
Mr. Little: And Dad.
Mrs. Little: We haven't told you the best news of all.
Mr. Little: You have a brother, named George.
Stuart Little: What do I call him?
Mrs. Little: George.
Only Hugh Laurie played a character both in the series and the movie.