When they’re sharing a stage, singing and playing jangling guitars, Susan Cowsill and Vicki Peterson exhibit a natural rapport.

They engage in friendly banter and, most of all, their voices blend seamlessly, like those of sisters.

The New Orleans-based Cowsill, a member of the 1960s family pop group the Cowsills, and Peterson, singer-guitarist in the ’80s pop-rock band the Bangles, have another musical identity: the Psycho Sisters.

Cowsill and Peterson are also sisters-in-law. Peterson married Cowsill’s older brother, John, in 2003. In the late 1960s, the future Bangle developed a childhood crush on John when she saw the Cowsills on TV. Simultaneously, Susan Cowsill inspired Peterson to reach for her musical dreams.

“My older sister said there was a girl in the Cowsills who was my age,” Peterson said from Los Angeles during a conference-call interview with Cowsill in New Orleans. “I freaked out. ‘Wait a minute. What? She gets to play in a band.’ Because that’s all I wanted to do, already, and there was this little girl my age on ‘The Ed Sullivan Show.’”

Peterson grew up and found music stardom with the Bangles. She later joined the Continental Drifters, which featured the girl in the Cowsills she’d envied all those years ago. And then later, she married that girl’s brother.

“There is,” the impish Cowsill noted, “something cosmic about this situation.”

Cowsill and Peterson have known each other since 1978. They toured Europe as the Psycho Sisters in 1992 and did more work together in the New Orleans-based Continental Drifters, which included the Dream Syndicate’s Mark Walton and the dB’s’ Peter Holsapple.

Twenty-two years after Peterson and Cowsill first performed as a duo, they’ve released the Psycho Sisters’ album debut, “Up on the Chair, Beatrice.” The duo’s record release show is 9 p.m. Friday, Sept. 12, at Chickie Wah Wah.

“Beatrice” includes three Peterson and Cowsill co-writes; a wistful Peterson-Bob Cowsill collaboration, “Never, Never Boys”; a gorgeous remake of the Cowsills’ classic “Heather Says”; and a charming take on the Harry Nilsson-penned Monkees song “Cuddly Toy.”

Occupied though Peterson and Cowsill were with various projects through two decades — including the Bangles, Cowsills, Go-Go’s, Continental Drifters and back-up singing for Belinda Carlisle, Hootie and the Blowfish and others — they never forgot about the Psycho Sisters. In 2012, they finally recorded their first duo album at Dockside Studio near Lafayette.

“The Psycho Sisters existed in our hearts for all these years,” Peterson said. “We finally found that little window of time where we could physically get together and make the album happen.”

Reaction to “Up on the Chair, Beatrice,” a work distinguished by multi-layered vocals, skillful pop songcraft and bright, Byrds-esque guitars, has been overwhelmingly positive.

“Not everybody is going to be a fan, and that’s totally cool and OK, but I haven’t seen one negative thing,” Cowsill said. “I think the spirit in which the album was made is being received in kind.”

Sixteen months apart in age, Peterson and Cowsill could, chronologically, be sisters. And Cowsill noticed their vocal blend the first day their voices intertwined.

“Susan kind of looked at me,” Peterson recalled. “She said, ‘You sound like my brother. This is weird.’ ”

Peterson’s biological sister and Bangles band mate, Debbi noted the blend, too.

“Debbi said, ‘You guys have that sister thing,’ ” Peterson said. “And the genetic vocal blend in the Cowsills, the Andrews Sisters, in all of those family groups, has always been one of my favorite sounds on the planet.”

“It is a gift, it’s magical,” Cowsill said of singing with her best friend, band mate and sister-in-law. “What I hear right next to me when she’s singing, it’s like flying.”

The Psycho Sisters

WHEN: 9 p.m. Friday, Sept. 12

WHERE: Chickie Wah Wah, 2828 Canal St.