After Pope Benedict XVI's announcement today that he will resign on Feb. 28, his older brother told The Associated Press that the pontiff had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.

Talking from his home in Regensburg in Germany, Monsignor Georg Ratzinger, 89, told the AP his younger brother, formerly Joseph Ratzinger, was having increasing difficulty walking and that his resignation was part of a "natural process."

"His age is weighing on him," Ratzinger said. "At this age, my brother wants more rest."

FULL COVERAGE: Pope Benedict XVI Resignation

Pope Benedict XVI was the oldest pope to be elected at age 78 on April 19, 2005, but according to a Catholic historian, the now 85-year-old pontiff never aspired to become pope.

Writer and historian Michael Hesemann spent months interviewing Monsignor Georg Ratzinger at his home in Regensburg to capture the intimate details of his life with the pope, from childhood to papacy. The two brothers have always been close.

These interviews became Ratzinger's memoirs in a book titled "My Brother, The Pope," which came out last March.

Hesemann, 48, is a German Catholic and an expert in church history. In writing this book, Hesemann told ABC News in an interview last year, he was surprised to learn that the pope was an "unambitious" man, "a loner" growing up, who was content to be a theology professor in Germany and never aspired to rise through the Catholic ranks.

"[The pope] never wanted to become a bishop, he never wanted to become cardinal, he never wanted to go to Rome, for three times he resisted the call of Pope John Paul II to Rome, but eventually he had to be obedient to the pope … and certainly never wanted to be pope," Hesemann said at the time. "It was against his plan. It was the plan of God but was not the plan of Joseph Ratzinger."

RELATED: Pope Benedict XVI Resigns: The Statement

Having written books about Pope John Paul I and Pope John Paul II, the Vatican granted Hesemann permission to write a biography about Pope Benedict XVI shortly after his election to the papacy in April 2005. It was after that the biographer said he developed a strong interest in interviewing Pope Benedict's older brother, Georg Ratzinger. After waiting almost six years, Hesemann said the monsignor agreed to have him record his memoirs.

Hesemann made it clear that this book, "My Brother, the Pope," was not commissioned by the Vatican, and was produced in a publishing house in Germany, not by the Vatican library. But the writer admitted that the Holy Father's personal secretary, Monsignor Georg Gänswein, did fact-check the book with Pope Benedict because Georg Ratzinger is almost completely blind and couldn't proofread it.

Long before he was named pope himself, Joseph Ratzinger had the ear of Pope John Paul II, and served as his close adviser. Hesemann said Pope John Paul II often sought Ratzinger's advice and the two would have collaborated on how best to offer the famous grand apologies Pope John Paul II made for the wrongdoings of the Catholic Church.

"In every step John Paul II made, you have one way or the other, an influence of the Ratzinger theology," Hesemann said. "John Paul II was a wonderful communicator but, as they say, he was not a great theological professor, he was not a teacher of theology, he was not a theological genius, and so he needed Ratzinger."

But having his brother named pope "shocked" Georg Ratzinger at first, not because he was jealous, Hesemann said, but because Georg knew it meant he would not be able to retire and travel with his brother as they both had planned.

"It was a shock for him, it was a shock. He was deeply depressed," Hesemann said. "He did not go to the phone for one day. His brother the pope tried to call him many times and in the end, eventually, the housekeeper went to the phone, and picked it up, and there's the pope on the other line. He wanted to talk to his brother, and so for him, he said, 'Oh, my God, I don't want to talk to anybody, this is the worst thing that could happen because now it's like I don't have a brother anymore, he won't have any time for me.'"

Hesemann said Georg would travel to Rome four times a year to spend time with the pope.

"He's happy to have him be the pope," Hesemann said. "[But] he is missing his brother. He would like to have his brother at his side."