"Where he's needed he will make himself available, but he will not want to want to intervene in the affairs of his successor," Georg Ratzinger, a German Catholic priest and musician, told the BBC.
Benedict, 85 -- who before his 2005 election was Cardinal Joseph Aloisius Ratzinger -- had been advised by his doctor to stop making trans-Atlantic trips, Ratzinger, 89, told the BBC.
Benedict had been considering stepping down for months, Ratzinger said.
"When he got to the second half of his 80s, he felt that his age was showing and that he was gradually losing the abilities he may have had and that it takes to fulfill this office properly," Ratzinger told the network from his home in the Bavarian city of Regensburg, Germany, located at the confluence of the Danube and Regen rivers.
His younger brother's decision to resign was therefore part of a "natural process," Ratzinger said.
Benedict cited advanced years and infirmity Monday when he made his surprise announcement of his decision to step down in a speech in Latin to a small group of cardinals.
Benedict is the first pope in six centuries to resign.
The pontiff told the cardinals he had examined his conscience "before God" and said he felt he was no longer up to the challenge of guiding the world's 1 billion Catholics.
Benedict plans to carry out his duties until 8 p.m. Feb. 28, Vatican spokesman the Rev. Federico Lombardi said.
He then plans initially to retire to the lakeside papal residence at Castel Gandolfo, about 15 miles southeast of Rome, the Vatican said, and would later move into a renovated monastery for "a period of prayer and reflection."
Benedict's "fidelity to maintaining the truth and clarity of the Catholic faith, to cultivating ecumenical and interfaith dialogue and in reaching out to inspire the next generation of Catholics have been great gifts to us all," said Boston Cardinal Sean Patrick O'Malley.
Vatican officials said they hoped to have a new pope in place by Easter, which this year falls on March 31.
Only two top Vatican cardinals were informed beforehand about the historic announcement, The Wall Street Journal reported.
The last pope to resign was Gregory XII, who stepped down in 1415 in a bid to end the Western, or Papal, Schism, which had reached the point where three men simultaneously claimed to be the pope.
To end the strife, before resigning, Gregory convened the Council of Constance, which deposed or accepted the resignation of the other papal claimants and elected Pope Martin V.
In the 598 years since then, the College of Cardinals has descended on Rome as a papal conclave to elect a new pope only after the death of the reigning pontiff.
Conclaves have been held in the Sistine Chapel since 1878.
A two-thirds majority vote of the 117-member conclave is required to elect the new pope, under rules set by Benedict in 2007.