Complex sentence: She had fallen in love when at college, but her parents disapproved of her beau, her father due to his being from "a prominent Yankee and abolitionist family" and her mother because he had no money and many debts.
Split into simpler sentences: She had fallen in love when at college, but her parents disapproved of her beau. Her father objected to his being from "a prominent Yankee and abolitionist family" and her mother to his lack of money and being burdened by many debts.