The custom that flourished, especially during the sixteenth and seventeenth centuries, by which a pope would name as his chief minister and most important advisor a nephew or similar relative who was elevated to the rank of cardinal and thereafter oversaw many of the most vital elements of papal administration. The practice was not invented in the sixteenth century, as papal nepotism had long been an established part of the pontifical court.
Pope Adrian IV (1154-1159), for example, named his nephew Boso to the cardinalate and put him in charge of Castel Sant’Angelo. Throughout the Middle Ages, it was common for a pope from one of the leading noble families to promote the interests of his house, but nepotism began reaching absurd heights toward the end of the fifteenth century with the accession of Alfonso de Borja y Borja as Callistus III (1455-1458). He made two nephews cardinals and worked to assist other family members with such vigor that at his death, the Aragonese who had profited from his generosity were driven from Rome. One nephew, Rodrigo Borgia, became Pope Alexander VI (1492-1503). He made his son Cesare Borgia a cardinal and surrendered to him vast powers over papal policy. Cardinal-nephew could be young chaps; in 1545, Ranuccio Farnese was made cardinal by Paul III at the age of 15.
“A Pope’s nephew dies twice; the second time like all men, the first time when his uncle dies.” – Cardinal Albani
The cardinal nephew in later years developed out of the need for the pope, usually old at the time of his election, to be assisted in the demands of office by a younger and more energetic assistant. Given the climate of intrigue that often pervaded Roman society in the period, the pope regularly turned to a promising young nephew, as relatives were slightly more reliable than scheming prelates who might be anxious to replace the reigning pontiff. As a brother to Leo XIII, Giuseppe Pecci became the last cardinal-nephew to date in 1879. The practise seems to have died out.