Complex sentence: Pius XII was seen as one of the great thinkers to assume the papacy in the 20th century and was also the ultimate insider; his family were descended from the Roman aristocracy, and his brother was a lawyer for the Holy See.
Split into simpler sentences: Pius XII was seen as one of the great thinkers in the papacy in the 20th century. He was also the ultimate insider; his family were descended from the Roman aristocracy, with his brother working as a lawyer for the Holy See.