Complex sentence: Scarlett is secretly scornful of Melanie Wilkes, wife to Ashley, who shows nothing but love and devotion towards Scarlett, and considers her a sister throughout her life because Scarlett married Melanie's brother Charles.
Split into simpler sentences: Scarlett is secretly scornful of Melanie Wilkes, wife to Ashley. The other woman shows nothing but love and devotion toward Scarlett, and considers her a sister throughout her life because Scarlett married Melanie's brother Charles.