Complex sentence: The English nobles protested the marriage of the King's sister to a foreigner of modest rank; most notably, Eleanor's brother Richard, 1st Earl of Cornwall rose up in revolt when he learned of the marriage.
Split into simpler sentences: The English nobles protested the marriage of the King's sister to a foreigner of modest rank. Most notably, Eleanor's brother Richard, 1st Earl of Cornwall rose up in revolt when he learned of the marriage.