Smiling and waving and looking mighty pleased with themselves, the American blue bloods sailed into a Westchester County courthouse Monday as if they owned the joint.

Robert F. Kennedy Jr.! Another brother (what’s his name?) Douglas! And, first thing in the morning, before court officers had finished their second cups of coffee, the family matriarch strolled up to make a rare public appearance — 85-year-old Ethel Kennedy!

Blinded by white teeth, nauseated by a palpable sense of entitlement exuded by this crew, I watched as the spectators parted like the Red Sea­ as the widow of slain Sen. Robert F. Kennedy passed them by.

The crowd milling about the courthouse grew silently reverential. I saw a man lunge for his cellphone camera. But by then, a court officer body-blocked Ethel and retrieved a wheelchair so she might be rolled safely inside by family friend Joe Armstrong, the former publisher of Texas Monthly magazine.

The glittering, pompous array of Kennedys, the closest thing this country has to a royal family — most of whose members have devolved into this cabal of rich, idle slackers — descended on the courthouse in White Plains to cheer on the biggest slacker of them all, 54-year-old Kerry Kennedy.

Looking slightly annoyed in a shapeless, pale gray dress, Kerry hugged her relatives as if she hadn’t seen them in years. But when strangers attempted to break into the lovefest, she just smiled, waved and walked away.

World traveler Kerry blew into town to stand trial for driving while ability impaired by drugs, a misdemeanor stemming from a wild ride she took in her 2008 silver Lexus SUV on Interstate 684 in 2012, smashing into a tractor-trailer truck while allegedly under the influence of the sleeping pill Ambien.

When she isn’t standing trial for drugged driving, Kerry trots the globe looking for children to save from slavery, gays to save from torment and political prisoners to save from torture as a professional human-rights activist.

But lately, her delusions of grandeur have crumbled into failure. Kerry flew off to the African nation of Uganda last month to persuade President Youweri Museveni not to sign into law a measure that criminalizes being gay and could put some homosexuals in prison for life. Monday, as her trial began, Museveni signed the law.

Her brother Robert F. Kennedy Jr., 60, is another activist (how does one get a job like that?) working to protect the environment. But his greatest claim to fame so far was persuading Gov. Cuomo — Kerry’s ex-husband — to ban hydraulic fracturing. The method of removing natural gas from the earth could reap untold millions for impoverished folks in upstate New York.

In recent years, the storied Kennedy family has veered from the family business of public service and developed a sideline of attending one another’s court cases.

In 1991, the clan came out en masse as William Kennedy Smith, a cousin of Kerry’s, was tried and acquitted of raping a woman in Florida after partying with his uncle, the late Sen. Ted Kennedy. Ethel’s nephew Michael Skakel’s 2002 conviction for a murder he allegedly committed in 1975 was another big hit for the family. So was his court appearance last year in which he was freed on $1.2 million bail and granted a new trial.

Kerry on Monday didn’t look frightened like an ordinary human. With a cheering section of Kennedy family members at her back, Kerry looked put-upon — as if she was above it all.

Had Kerry taken her lumps, had she simply admitted that she made a grievous mistake when she got behind the wheel, we wouldn’t be here. Chances are that Kerry would be back in the planet-saving business, without having to worry about such trifling matters as the welfare of motorists in Westchester County.

In his opening, Kerry’s lawyer was quick to remind jurors she’s the daughter of Robert Kennedy and niece of slain President John F. Kennedy.

She adds tarnish to the Kennedy name.