NEW YORK — If you think there are some stories that simply can't be set to music, well, try this one on for size: a pair of upstanding black sisters in 1890s Texas pass for white — and become bank robbers.

Now, imagine this is not simply one of those middle-of-the-night epiphanies that flickers on out of nowhere, but is rooted in fact. Or rather, in the legend passed down from one generation to the next in a black American family that revels in the mystery of how much of it — if any — is true.

Which is why it may be just right for the stage.

Taking under its developmental wing a fresh young musical-writing team, Signature Theatre has opened its main stage to the world premiere of “Gun & Powder” by librettist-lyricist Angelica Chéri and composer Ross Baum. Because of the intriguing origins of the piece and some of the seasoned participants in the project, led by Broadway-tested director Robert O’Hara (“Slave Play”), the interest in “Gun & Powder” extends far beyond the audience in Northern Virginia’s Village at Shirlington. It already has commercial producers attached to it, a sign that hopes run high, and the pressure to prove the work’s mettle is considerable.

Taking place in small-town East Texas in 1893, “Gun & Powder” explores the lives of sisters Mary and Martha Clarke at a crossroads in American history rarely brought to life on a stage.

“You don’t have black and brown bodies in a musical set in the late 19th century,” says O’Hara, a writer-director whose own plays (“Bootycandy,” “Antebellum”) have been produced at Washington’s Woolly Mammoth Theatre, and many other places. “I often say people think of black and brown bodies in slavery, and then in civil rights. And nothing in between.”

In between is this mythic tale, dramatized by 16 actors in 25 songs, of two black women in petticoats and holsters, portrayed by Solea Pfeiffer and Emmy Raver-Lampman. The mere act of illumination — of declaring that these lives, lost to time and certainty, are worthy of creative investigation — speaks to both our shared love of history and the intense modern desire for confirmation of our ancestry. It raises nuanced questions about race and identity, the notion of being black but looking Caucasian. The added transgressive dimension of two unlikely bandits, a Lone Star State Bonnie and Cly . . . — er, Bonnie and Bonnie — makes the material seem all the richer.

For Chéri, a 32-year-old California-born playwright trained at Columbia and New York universities, the endeavor is personal and deeply emotional. This is her first musical, and it’s her own family’s folklore she’s mining.

“There is a sense of pride that comes in a retelling,” she explains. “It says we have something of legendary size in our family. This is our corner of American history.”

At a time when ownership of history plays out so provocatively in American politics as well as culture — in the divisive battles over immigration, in the “Hamilton”-style recasting of the American Revolution — “Gun & Powder” could be writing another compelling chapter.

“The moment she told me about her great-great-aunts being these notorious lady outlaws in the Wild West, that just sung to me,” says Baum, 29, who met Chéri in the graduate musical theater-writing program at NYU.

Of course, whether Mary and Martha really were criminal masterminds was impossible to pin down. Chéri grew up with her grandmother’s stories of the twin Clarke sisters. The pages of an old photo album contained a portrait of two prim-looking ladies with light features, about whom some relatives swore they knew the scandalous truth. Others in her family expressed skepticism.

“They were so beautiful and unassuming,” the writer says over breakfast in a Manhattan restaurant, during a break from auditions for another of her forthcoming plays, “Berta, Berta,” at Baltimore’s Everyman Theatre. She has relatives, she explains, “who are absolutely convinced they were bank robbers. But there are so many conflicting narratives, it’s hard to ascertain what actually happened.”

She understands from long, hard work how the veils of time — and a family game of telephone — can intertwine fact and fiction. She says she’s done as much research as she could; learning, for instance, that Mary and Martha had a white French grandfather and that other ancestors were enslaved. The story that she and Baum have written and rewritten over the past five years — including more than 90 songs whittled down to about two dozen — draws on both real and unverifiable events. No newspaper articles exist that might have confirmed the Clarke sisters’ exploits.

Centrally, though, Chéri evolved a portrait of independent-minded women living in the shadow of slavery, but prefiguring a time when black women might have access to a world that recognized them as equals. “The ancestral myth is that they would never go anywhere without being armed,” she says. What would give such a family memory meaning? “It was all about survival, and empowerment.”

The outsize legend allowed the songwriters to make the story completely their own.

“Because it’s a myth, it gives us a lot of leeway,” Baum says. “We’ll have like a sexy rhythm and blues number, a John Legend-inspired jazzy number, then it’s this uptight waltz number, then a Beyoncé-meets-Carrie Underwood kind of number. It’s all meant to feel organic to our storytelling.”

And the years of workshop productions and readings led to buzz and encouragement. Joe Calarco, until recently the head of new works for Signature, picked “Gun & Powder” out of the submissions to be worked on in the company’s summer program in 2017.

“It’s a very unique story told in a thrilling way,” Calarco says. “She’s a beautiful writer, a playwright first, and it shows in the writing of the book; her lyrics are smart and surprising. And Ross’s score is just gorgeous.”

The following year, after Baum and Chéri won the Richard Rodgers Award for musical works in progress, Calarco recalls saying to Eric Schaeffer, Signature’s artistic director: “Eric, if someone else does the world premiere of this show that we developed, it will break my heart.”

For O’Hara, whose work has revolved for so long around defining theater as a forum for black stories, “Gun & Powder” is not a musical about passing as someone else. “It’s about two women who step into what white privilege means,” he says, “and the dangers of white privilege even then.”

This idea of blurred lines of identity seems such an apt starting point for a musical that was, in essence, born out of such a fuzzy bit of personal history. Peeling back the layers of time and poring over the archives of the unknowable have only added to Chéri’s pleasure at seeing it all come to the stage. And she’s excited for her own family’s other tale-spinners to come see what she and Baum have added to the legend of Mary and Martha.

“They were not who people thought they were,” she says. “They were already in disguise.”

Gun and Powder, book and lyrics by Angelica Chéri, music by Ross Baum. Directed by Robert O’Hara. $40-$85. Through Feb. 23 at Signature Theatre, 4200 Campbell Ave., Arlington. 703-820-9771. sigtheatre.org.