Complex sentence: Grace begins to fall in love with Charles/Augustus in spite of herself and so, when Lady Gay interrupts their courtship, Charles easily persuades her to distract Sir Harcourt from marriage to Grace by apparently accepting his affections.
Split into simpler sentences: Grace begins to fall in love with Charles/Augustus in spite of herself. When Lady Gay interrupts their courtship, Charles easily persuades the lady to distract Sir Harcourt from marriage to Grace by apparently accepting his affections.