Brian's Sister is the younger sister of Brian Johnson and younger cousin of Kendall. She was only seen with Brian and their mother at the start of the film. It is also revealed that those portraying them are half-siblings in real life (by having the same real-life mother, but different fathers). Her only line in the whole movie was when their mother told Brian to find a way to study and his sister annoyingly said, "Yeah!" It is revealed that she hates him.
She is portrayed by then-7-year-old Mary Christian, who was Anthony Michael Hall's real-life half-sister.