Despite its title’s promise of eroticism, “The Foursome” is about as un-kinky as a book can get.

Christina Baker Kline’s novel was inspired by the true story of Chang and Eng Bunker, the conjoined twins from Thailand who became a sensation when they toured the U.S. in the 1800s. The twins have been the subject of several books, both nonfiction (“Inseparable,” by Yunte Huang) and fiction (“Chang and Eng,” by Darin Straus). But Kline takes a different tack, focusing on the Bunkers’ marriages to North Carolina sisters Addie and Sarah Yates.

The appeal of the book, which is narrated by Sarah, is how skillfully Kline straddles the divide between the rarity of the conjoined brothers and the conventionality of their marriages. “Here’s the truth,” Sarah writes at the beginning. “Even the most extraordinary life feels ordinary when you’re living it.”

Kline has Sarah, who is sheltered even by 19th century standards, connect the extraordinary world of the brothers with the huge changes brought about by a marriage, any marriage: “How strange marriage is even in the most ordinary of circumstances. One day you’re living separate lives, in separate houses, barely allowed to speak or touch, and the next you’re yoked together for life.”

There’s nothing prurient about the way she does it but Kline imagines for us the details of life with conjoined twins: sleeping arrangements (each sister had her own room but would go to their brother’s side of the bed on alternating days), communication (Sarah finds that the only way to tell Eng something without Chang knowing is via secret notes), even the work on the families’ farm, where the brothers’ two-ax method of chopping down trees is the envy of their neighbors.

Kline provides just enough of that detail, along with references to rapidly moving current events, to convince us that she knows what she’s talking about (and, in fact, the Yates sisters are distant ancestors of hers). But this is really Sarah’s book.

The author of historical fiction including “Orphan Train” and “A Piece of the World” occasionally veers into modern-day psychoanalysis of her characters. But, for the most part, her portrait of Sarah convincingly captures what it would be like to be a woman with few options, a woman whose family has enslaved people and who is beginning to understand how wrong that is (partly because she herself feels constricted), a woman who is learning that her entire life was built on the backs of people she barely noticed.

Little is known about the Yates sisters, so most of what we learn is invented by Kline. She has more to go on with the brothers, who were among the most famous people in the U.S. for a time, but the behavior she imagines for them also is compelling — helping us understand, for instance, how brothers who were exploited by P.T. Barnum and others could turn around and exploit the humans they bought and sold. (For more on how the peculiar institution of slavery twisted people’s morals into knots, I highly recommend Edward P. Jones’ monumental, Pulitzer Prize-winning novel, “The Known World.”)

By the end of “The Foursome,” Sarah realizes that the life she has characterized as “ordinary” was anything but. Kline depicts good times and bad times for her main character, who learns a lot over the course of the decades covered in “The Foursome” and who, after the Civil War, finds herself well-prepared for a brave new world.

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The Foursome

By: Christina Baker Kline.

Publisher: Mariner, 368 pages.

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