Complex sentence: When their father finally arrives, everyone is surprised when he introduces his new girlfriend Amy (Alicia Witt), who is not only young, but also the actress playing Cheri in the film adaptation of the book.
Split into simpler sentences: When their father finally arrives, everyone is surprised when he introduces his new girlfriend Amy (Alicia Witt). Who is not only young, but the actress playing Cheri in the film adaptation of the book.