Complex sentence: Lupin's fiancée, Daisy Mutlar, is the sister of one of his theatrical friends and is, he says, "the nicest, prettiest, and most accomplished girl he ever met".
Split into simpler sentences: Lupin's fiancée, Daisy Mutlar, is the sister of one of his theatrical friends. She is, he says, "the nicest, prettiest, and most accomplished girl he ever met".