Complex sentence: She was one of four daughters of the Swedish-born artist Anthony Rubens Montalba, all of whom achieved considerable artistic success, though Clara was arguably the most accomplished and best-known of the four.
Split into simpler sentences: She was one of four daughters of the Swedish-born artist Anthony Rubens Montalba. She and her three sisters all achieved artistic success, though Clara was arguably the most accomplished and best-known of the four.