She’s a little bit perky. He’s a little bit cynical.

No, that’s not quite right. She’s very perky, a fireball of sass and musical laughter and positive thinking. He’s really more wry than cynical, witty and reflective about his bumpy ride in show business. So please excuse the shorthand: It’s hard not to fall into well-worn rhythms when contemplating two stars whose longevity makes them feel as familiar as your own family. The most adorable brother-sister team in history have their own well-worn rhythms that enhance this illusion for you.

“When we came to Las Vegas, there were a lot of naysayers,” Donny Osmond is saying about their current gig, six years and counting in a Sin City stage spectacular. “That’s why it was a six-week contract.”

“I don’t know that there were a lot of naysayers,” Marie Osmond interrupts. “That sounds very negative.”

Donny: “Oh, yes, there were. ‘I don’t know if Vegas is ready for Donny and Marie.’ ”

Marie [to a reporter]: “I don’t agree with him.”

Donny [defensive]: “Hey, I talked to them.”

Marie: “Well, so did I! I did my due diligence too! . . . I’m not out to prove anything. I’m grateful to still be here working.

Donny: “I’m out to prove something. Every time I step onstage.”

Marie: “Ya think? Holy Hannah! Why do you think we took a 28-year pause? I can only take so much!”

The bicker-banter keeps on coming in this green-room interview, just as it did on their 1970s TV show, just as it does five days a week on the stage of the Flamingo Las Vegas Hotel and Casino — and just as it will when they bring a Christmas version of their show to the National Theatre for a six-night run beginning Tuesday.

It’s an act that, for all their nods to modern pop and timeless tunes, trades openly on nostalgia, harking back to the all-but-extinct variety-show format they were born for, reminding fans of their days as squeaky-clean teen idols singing and doing slapstick in satin bell-bottoms — a reputation that threatened to sink them as they tried to make their way, solo, as adult performers. They sing solo and they sing together, through a blur of glitzy costume changes and high-energy backup dancers and dry-ice fog. No more cheesy skits, sorry, but there’s plenty of that highly entertaining passive-aggressive sibling badinage. (“We have this unique sibling chemistry,” she says, “like an acid peel.” Cute, Marie, real cute.)

After years of running from their
Donny-and-Marieness, Donny and Marie Osmond are finally owning it.

“Hey, I sing ‘Puppy Love’ out there on every show,” Donny says. “I don’t know if I could have done that kind of show in my 20s or 30s. . . . But I have embraced who I am.”

They look great, of course. Clear eyes, unlined faces. Some blessed combination of Mormon clean living, Hollywood skin care and the babyface gene. Marie, 55, turns up for a photo shoot dressed as if for a Grand Ole Opry cocktail party, flashy but buttoned-up in curve-hugging red and black, fishnets, towering heels, a torrent of raven hair. Donny, who turns 57 next week, dons the cool camouflage of a recording-industry elder statesman, a trim-fit charcoal sweater and pants. Living the theme song after all these years: She’s a little bit country, he’s a little bit rock-and-roll. (So there, it’s been said.)

They are some of the most relaxed interview subjects you can imagine, but they have been doing this a long time. Before they made their debuts (he at 5, she at 3), they watched as four of their older brothers — Alan, Wayne, Merrill and Jay — established the Osmond brand as the cute kids harmonizing barbershop-style on “The Andy Williams Show.”

Donny joined the act, and after a few years, the brothers hit it big as an R&B-inflected pop act, friendly rivals to the Jackson 5; then the group pivoted into a full-fledged rock band, singing hits such as “Crazy Horses” in spangled Elvis suits for arenas filled with screaming, fainting girls. Marie launched a career in country music at 12 — already at that age, she says, she sensed the pop world was brutal on young women — and scored a No. 1 hit, “Paper Roses,” at 13. Then, suddenly, the older Osmonds receded into marriages and behind-the-scenes roles, and it was all about Donny & Marie.

The transition may have felt seamless to the young teens at the center of it, but “it was a strategic move” by the family’s management, recalls Jimmy Osmond. The clan’s youngest, who had his own pre-adolescent performing career (he’s still huge in the U.K.), notes that “Donny was the right age at the right time. . . . It was the next best thing to move the brand forward.”

A series of brother-sister bubblegum-pop duets followed — and then, in 1976, their own Friday-night variety show on ABC. She was 16, he was 18. A mix of musical performances, ice-skating chorus lines, hokey comedy and celebrity guests, it may have been Generation X’s first appointment television. Like all hit shows, it was a huge hit until it wasn’t.

When the show was canceled in 1979, the Osmonds were broke. The family had poured its fortunes into an elaborate TV studio in Orem, Utah, closer to home.

“We lost the plot and tried to become big producers,” says Jimmy, who now oversees some of the family’s business interests. (He recently penned a semi-autobiographical children’s book, “The Awesome Possum Family Band.”) “We’d spend more money [on the show] than we’d make, because we cared so much about the show.” Their parents, George and Olive, refused to file for bankruptcy. Jimmy believes the years they spent digging out of debt made them stronger as a family, eventually.

But Donny entered the ’80s adrift. The culture turned on him: His Broadway debut closed on opening night, he couldn’t get a record contract, and he found himself playing ever-smaller rooms. Some advised him to rough up his goody-goody image with a bogus drug scandal; others told him to change his name. He grappled with crippling social anxiety — a common disorder that makes its victims paranoid that everyone is watching and criticizing them, although for him, that was no delusion.

As a man who spent decades reinventing himself — a return to the top of the pop charts in 1989 with “Soldier of Love”; a long, successful stage run in “Joseph and the Amazing Technicolor Dreamcoat” — he now understands the curse of the teen idol. “When you hit it so big at, say, 13 years old, and all these 11-, 12-year-old teenyboppers love you, you have about a five-year span, maximum. Then you’re going to be 18, and 18-year-olds consider themselves adults — and whatever they liked back then belongs back in that era. . . . So they kind of throw you out with adolescence.”

He adds: “Had I not hit it so big as a teen idol, and I would have just established myself as a theater actor or a musical entertainer, it would have been a lot different. But I probably wouldn’t have the audience I have today.”

Home life was a solace. He has been married to Debra, a fellow Mormon, for 36 years. They have five sons — the grown ones in careers such as finance and physical therapy, not show business — and seven grandkids.

For Marie, who moved on more easily to TV movies, Broadway and country radio, her career was the solace. She grappled with a divorce at 25, a bruising case of postpartum depression, the collapse of her second marriage after 21 years and, in 2010, the suicide of her son Michael.

“A lot of people criticized me for going right back into the [Vegas] show,” she says. “But I had seven other children. I had to show them they had to keep living.”

She and her brother launched their Christmas shows that year, in part because, in her grief, she couldn’t bear to be home for the holidays. The next year, she remarried her first husband, Stephen Craig. Working on a memoir during these busy years, she kept having to rewrite it.

“My life has been interesting,” she says. “People are like, ‘You’ve written your whole life,’ and I’m like, ‘No, no, no, I haven’t.’ ”

Can they imagine an alternative reality? What their lives would have been like without a head start in show business?

Donny, the reflective one, draws a blank. Sighs. “Isn’t that terrible?” No idea. “There wasn’t a lot of choosing in my life. By the time I was 5, I was locked in.”

Marie has some notions. “Maybe some kind of education. A teaching degree or something.” Donny, she says, “would have been a tech nerd. He would have been designing software somewhere.”

But let’s go back for a second. Donny and Marie, can we talk about your old show? Really talk? What do you make of it now?

“Ah, I love it,” Donny says. “The comedy was slapsticky and silly, but that’s what we did back then.”

“The clothes were stupid, and the hairdos were ugly,” Marie says, “but it was all good back then.”

Go to YouTube and see for yourself. The first moments of that first episode take you straight to the heart of the 1970s, some depth of garishness and smarm unleavened by irony, which possibly hadn’t been invented yet. Donny and Marie are disco-dancing in red-and-white satin outfits. They are heart-piercingly beautiful — huge brown eyes, gleaming white teeth, touchingly young. And they are singing the old K.C. and the Sunshine Band song: That’s the way, uh huh, uh huh, I like it, uh huh, uh huh.

It is just plain mortifying.

Do they remember this?

Donny chimes in with the song: “I like it! Uh huh, uh huh!”

“Yeah, I remember that!” Marie says brightly. Their costume designer was really just a set designer with a can of spray paint, she explains. That’s why the clothes were so absurd.

But — don’t you know that’s a song about sex?

“Oh, yeah,” Donny says.

Did you not realize that at the time?

“Oh, we had no idea!”

“You know, listen to the radio these days,” Marie says, “you can take songs a million different ways.”

“That was the popular song then,” Donny says. “Did you know it was about sex?”

Well, not back then!

“Exactly!” Marie laughs.

They reminisce some more about the show. The three days of prep and two days of taping it took to put together each episode. Milton Berle, Lucille Ball, Super Bowl quarterbacks, the cast of “Star Wars,” with their older brothers playing dancing storm troopers. How they got Lee Majors, then the “Six Million Dollar Man,” to appear by offering a role for his wife, who turned out to be Farrah Fawcett.

Marie: “She walked out, and I’m like this 14-year-old girl, and I’m like, ‘Crap, I got to stand by her?’ ”

Donny: “I had a poster of her on the wall in my bedroom.”

Marie: “From Jaclyn Smith to Raquel Welch, we worked with these incredible women. My first duet was with — ah, my brain, not Tom Jones . . .

Donny: “Engelbert Humperdinck?”

Marie: “No, it wasn’t Engelbert. I’ll tell you in a second.”

Donny: “Not Andy Gibb?”

Marie: “No. He’s beautiful. I’ll tell you in a minute . . . Kris Kristofferson! And I remember thinking, what. ’Cause l was like 15.”

Donny: “And then I had to do a duet with Cheryl Ladd, and I was like, what.”

Marie: “Hahahahaha! ‘Now be romantic.’ Sure!”

That was the Age of the Osmonds — the 1970s, a very different era from our own. “Everybody took chances then,” Donny says. “Everbody is so scared of making the wrong move now, and everything is so calculated, and it’s not real. Everything’s so Autotuned. Back then, it was like: ‘Sing it!’ ”

Marie: “Sing it, and either you could do it or you couldn’t. . . . People were saying the Osmonds weren’t cool. When did that happen? Because we sure had a lot of people follow us! Hahahaha. I think it’s sometimes people who were so afraid to say they liked it, that they wouldn’t be considered cool.

Donny: “It’s a dynamic that, I don’t even know how to explain when you say the Osmonds weren’t cool, because we weren’t, in certain circles — but back in the closet, we were really cool.”

They both laugh at this. He adds: “Now they can come, proverbially, out of the closet and say, ‘We used to like the Osmonds.’ ”