Complex sentence: He falls in love with Marianne at first sight as she reminds him of his father's ward whom he had fallen in love with when he was young but was prevented from marrying her because his father was determined she marry his older brother.
Split into simpler sentences: He falls in love with Marianne at first sight, as she reminds him of his father's ward whom he had fallen in love with when he was young. He is prevented from marrying the ward because his father was determined she marry his older brother.