Complex sentence: A rival group of older artists, The Clique, used its influence against the Pre-Raphaelite Brotherhood and its principles were publicly attacked by the President of the Academy, Sir Charles Lock Eastlake.
Split into simpler sentences: A rival group of older artists, The Clique, also used their influence against the Pre-Raphaelite Brotherhood. Their principles were publicly attacked by the President of the Academy, Sir Charles Lock Eastlake.