Priscilla L. Buckley, who became known as the den mother of modern American conservative intellectuals through her decades-long stewardship of National Review, the influential magazine founded by her brother William F. Buckley Jr., died March 25 at her family’s estate in Sharon, Conn. She was 90.

The cause was kidney failure, said her nephew Christopher Buckley.

Miss Buckley was working in the Paris bureau of the former United Press wire service in the mid-1950s when her brother called her up and persuaded her to move back to the States and join his fledging publication in New York.

National Review, he hoped, would provide a withering response to the liberalism that he believed had come to dominate the American political scene. It would, as the editors wrote in the magazine’s inaugural issue, stand “athwart history yelling Stop.”

William Buckley knew that his sister — a millionaire’s daughter who had thrived in both the elite schools of Europe and in the liquor-soaked newsrooms of the wire service — had just the combination of erudition and grit to make his vision a reality. (She dressed like a lady, but was known by her nickname, “Pitts.”)

With his sister as his steady-handed managing editor from 1959 to 1986, William Buckley was credited with founding and shepherding the modern conservative movement in America. Miss Buckley was the “flute in our conservative orchestra,” columnist George Will once remarked, “ who taught, by example, the compatibility of political commitment and generosity of spirit.”

In the beginning years, National Review offered an outlet for writers including Whittaker Chambers, John Chamberlain and James Burnham. Later, with leadership from Miss Buckley, it nurtured writers including Paul Gigot, Richard Brookhiser and Mona Charen.

The ragingly brilliant and charming William Buckley may have enticed those thinkers to National Review, observers have noted over the years, but Miss Buckley enforced deadline and saw their copy to print. According to a long-running joke, National Review was “Miss Buckley’s Finishing School for Young Ladies and Gentlemen of Conservative Persuasion.”

Miss Buckley’s office, the New York Times reported, was directly below her brother’s. They shuttled copy back and forth to each other using a dumbwaiter. When he was away on travel or otherwise occupied with his writing or speaking engagements, Miss Buckley was effectively in charge of the publication.

Linda Bridges, editor at large of National Review, recalled in an interview that William Buckley was sailing in the Aegean when Robert F. Kennedy, the U.S. senator and presidential candidate, was assassinated in June 1968. The magazine was about to go to press — with a cover story attack on Kennedy, a Democrat — when the shooting took place.

In a span of hours, Miss Buckley saw to it that the article was replaced with another one, and that other than an editorial on the shooting, all references to Kennedy were expunged from the magazine.

She became known for her grace under the pressure of deadlines and over the years handled more than a few “C.M.S” editions — the initials standing for “cancel my subscription.” These included the ones when her brother called for the legalization of marijuana and when other writers denounced the John Birch Society, a far-right organization.

Once, in the early years of the magazine, she joined other editors in playing a prank on Arthur Schlesinger Jr., the historian who wrote extensively on liberalism in America. When National Review held a contest asking subscribers to predict the outcome of an election, Schlesinger came in 11th place and the editors had a donkey shipped to him as a prize. Schlesinger sent it back.

“We tended to prove that conservatives need not be kooky, crazy, racist, isolationist, or on the borders,” Miss Buckley once said. “And I think our most important weapon was humor.”

After her retirement in 1986, Miss Buckley wrote two memoirs: “String of Pearls” (2001), about her years with United Press, and “Living it up with National Review” (2005).

Reviewing “String of Pearls” in The Washington Post, critic Jonathan Yardley called Miss Buckley “a writer of real skill and accomplishment” despite her decades laboring “in the comparative obscurity of the managing editor’s desk at the family magazine.”

Priscilla Langford Buckley was born Oct. 17, 1921, in New York, the third of 10 children born to a millionaire oil investor.

Her early schooling was in France and England. She graduated from the private Nightingale-Bamford School in New York and in 1943 from Smith College, where she was managing editor of the student newspaper.

Miss Buckley then joined United Press for $18.50 a week. Like her brother, she worked briefly for the CIA — a job she called “paper-pushing” — before returning to UP in 1953 to become the Paris correspondent.

Then her brother called, inviting her to come write for National Review. “When Bill phoned me,” she told an interviewer, “he said that he had an awful lot of professors on the magazine, but he didn’t have a single working journalist.”

She agreed, on the condition that she could have six weeks of vacation a year to pursue her hobbies, which included sailing and quail hunting.

After her retirement as managing editor, she remained with the magazine as a senior and contributing editor. She wrote for, among other publications, the New York Times, Harper’s Bazaar, Cosmopolitan and Shooting Sportsman.

Miss Buckley never married or had children. Survivors include two brothers, James L. Buckley, a former U.S. senator and federal judge, and Reid Buckley; and a sister, Carol Buckley.

Her brother William died in 2008 at 82. He always remained Miss Buckley’s kid brother.

“When you’re 12,” she once joked in an interview with the Litchfield County Times, “what an 8-year-old says has absolutely no importance in your life. And it stays that way even when you’re adults.”