“If you don’t tell their story, who will?”

This was the question posed to Christina Baker Kline by Lesley Looper, a cousin and Duke University librarian, about the lives of the renowned “Siamese twins” Chang and Eng Bunker and their wives, Sarah and Adelaide Yates — Kline’s distant relatives.

The short answer is that a lot of people have. The famous brothers, conjoined at the chest, who came to America in 1829 and eventually settled in North Carolina, have been satirized in poetry, made cameo appearances in works by Herman Melville and Mark Twain, been used as a metaphor during the War Between the States, and been the subject — or at least the literary device — of 21st-century musicals, plays and movies. Does the fact that Kline’s genealogical family tree includes them make her imaginings somehow more prescient? Since the twins died 152 years ago, probably not. What is quite clear from the earliest pages of Kline’s The Foursome, due out this month, is that she has taken extraordinary care to imagine her characters less as curiosity and more as men and women in full, portrayed with distinct traits, virtues and flaws, and very much creatures of their age, one of America’s most turbulent times.

Here’s a Wikipedia-worthy primer: Chang and Eng were brought to the United States from Siam (today’s Thailand) by the Scottish merchant Robert Hunter and a sea captain named Abel Coffin, who put them “on tour” in Britain and America. The on-again, off-again business wound up a decade later with the brothers touring on their own with their own staff, becoming wealthy in the process. In July 1839 they made an appearance in Jefferson, North Carolina, and in October of that year, they returned to purchase 150 acres in Wilkes County, where they would meet and marry the Yates sisters. This is where the novel takes over.

When Kline realized that Sarah (Sallie) was not buried in the same resting place as Chang, Eng and her sister, Adelaide, she discovered the voice of her narrator. Sallie is as clear-eyed about herself as she is every other character in the novel. “Addie possessed the self-assurance of the beautiful. She was used to being seen, and it made her bold about being heard,” writes Kline. “I inherited our mother’s round cheeks, her solid bones and small gray eyes, her unruly auburn hair. Addie took after Papa’s family: tall and lean, with dark-fringed lashes and high cheekbones. She shone in contrast to my ordinariness. She was charming while I was shy.”

The vivacious Addie is drawn to Chang, the more dominant brother. “Addie claimed she’d fallen in love with Chang, and maybe she had. She said she felt it deeply. But Addie felt everything deeply,” writes Kline. “Somehow, though I’d voiced my misgivings from the beginning, I’d let the months unspool without taking a firm stand. Now I found myself swept up in my sister’s insistence that marrying the brothers was the right, the only, thing to do.”

Kline doesn’t shy from the physical awkwardness of this union squared, though neither does she dwell on it. The mantra for Sallie is compartmentalization. Don’t think about everything, “only the next thing.”

The sisters’ conversation on their wedding day is portrayed like this:

“Everyone will be staring at us,” I whispered.

“Of course they will. We’re the brides.”

“They’re thinking about — about tonight.”

“Don’t be silly. Nobody’s thinking about that, except maybe you. You’ll be fine. Remember: only the next thing. All right?”

“All right.”

The foursome marries in 1843. After finessing the physical, Kline does an admirable job of portraying these two families through the next 30-plus, turbulent years, through war, peace, the inevitable loss of parents, the birth, and sometimes tragic death, of children and the eventual death of Chang and Eng. In fact, it is this dramatization of the travails of two families that, in a way, normalizes that which is anything but. The couples eventually live in separate houses, one in Surrey County, one in Wilkes County, spending three days at each. “During the three days in the home of the host, the visiting brother will conduct no business and express no opinions. He is to be a silent partner,” declares Chang. Between them the two families would have 21 children who would grow into an assortment of cousins devoted to one another.

Though joined at the chest, the brothers are not the same person. “Eng liked to gamble, his eyes brightening with each new hand. Chang preferred to drink. Neither quite approved of the other’s vice.” Chang could be cruel and moody, Eng the peacemaker. “Eng’s instinct was to ignore or concede, but even he had his limits. Sometimes, like a cat poked too often, he struck back. More than ever, I saw how tightly the band bound him to his brother. What had once been a tether now felt like a shackle.”

Every time their financial picture darkens, the brothers go back on the road to refill the coffers, but the way they are perceived has changed. What once was a curiosity has given way to ridicule. They eventually hook up with P.T. Barnum, who dislikes the brothers because of their independent streak as much as they detest the famous showman for his exploitation.

Chang and Eng are free men of color who become slaveholders and supporters of the Southern cause. Two sons, one from each family, fight for the Confederacy. “The brothers had learned early on that the world is divided into those with power and those without. Those who own and those who are owned. They’d decided — perhaps from the moment they first felt the weight of coins in their palms — where they stood on that divide.” The families feel the depravations of war and struggle with issues of race. “The shortages deepened. Every stitch of fabric was repurposed, every scrap of food stretched.” Stoneman’s cavalry came. The world changes, the enslaved are enslaved no more. “The hardest part wasn’t learning to do things for ourselves, though that was difficult enough. It was learning to see people we’d spent years looking through. To acknowledge that the women who had wiped our children’s tears had children of their own whose hurts had gone unseen.”

If the world paid attention to Chang and Eng, Kline gives more than equal time to Sallie and Addie and the place of women in the 19th century, dramatized throughout, from unwanted pregnancies at the hands of unscrupulous men; to Eng, the slaveowner taking advantage of the enslaved Grace; to the assured figure of Sallie’s lesbian aunt, Joan. Given all that, The Foursome stretches beyond the voyeuristic, attempting to paint a fuller picture of two brothers and two sisters, tethered by more than just flesh.