When God asked Cain if he knew where Abel was, the first murderer gave the first shifty answer: “Am I my brother’s keeper?” Together, Cain and Abel had made up fully half of the world’s population. Was it really that unreasonable for God to have expected that they kept close tabs on each other?

In “Big Brother,” Lionel Shriver’s 11th novel, thick-skinned sibling tension is constantly flipped over to reveal its tender underbelly of devotion. The author, whose “We Need to Talk About Kevin” (2003) depicted a teenage sociopath for whom the bonds of family were about as strong as worn-out rubber bands, has clearly been thinking a lot about filial ties, debts owed and sins unconfessed.

In Shriver’s newest book, a 40-year-old woman’s decision to put her marriage on hold so that she can help her morbidly obese brother lose weight is cast as an act both responsible and irresponsible, caring and selfish. And in her narration, we find a frank rejoinder to Cain’s grumbled abdication of responsibility: “What is wonderful about kinship is also what is horrible about it: there is no line in the sand, no natural limit to what these people can reasonably expect of you.”

Are you your brother’s keeper? Damn straight you are, whether you like it or not. When Pandora Halfdanarson’s older brother, Edison, lands in Iowa, he’s nothing like the rakishly handsome, semi-famous New York City jazz pianist she last laid eyes on four years ago. For one thing, Edison has added about 220 pounds to his 165-pound frame, necessitating his removal from the airplane via wheelchair. But there’s evidence of depletion as well as expansion. For a man so heavy, her brother seems utterly adrift. From what Pandora can gather, he has few friends, no romantic attachments and no permanent home. His musical career has dried up. His self-confidence has hardened into self-aggrandizing nostalgia, shot through with bitterness.

Pandora and Edison grew up in Los Angeles as children of a Hollywood B-lister afflicted with an A-lister’s solipsism; their own recollections of their aloof father clash with his beloved TV-show persona as a sweet, caring dad, fully engaged in the lives of his fictional TV kids. The two siblings grew even closer when their mother died — maybe it was an accident, maybe it was a suicide — and they had no one but each other to turn to for comfort. As they developed into very different kinds of adults, Pandora’s devotion to her brother never waned, even as they grew more distant. “It was Edison from whom I first learned loyalty; it was therefore Edison from whom all other loyalties flowed,” she observes after making arrangements for his stay.

”Big Brother” by Lionel Shriver. (Harper)

Even before Edison had morphed into a gargantuan figure, Fletcher, Pandora’s furniture-making husband, was prone to recoil from his brother-in-law. But now Edison’s boorish behavior quickly pushes Fletcher to his breaking point. When one of Fletcher’s prized chairs reaches its own breaking point under Edison, the friction between the two men Pandora loves most turns to open hostility. While helping her brother pack his belongings, she makes a fateful decision: She will rent an apartment nearby for them to share while she coaches him back to health with the aid of a liquid diet and her own unbending will.

As a writer, Shriver’s talents are many: She’s especially skilled at playing with readers’ reflexes for sympathy and revulsion, never letting us get too comfortable with whatever firm understanding we think we have of a character. Edison’s abrasiveness and self-pity are every bit as off-putting as his physical appearance, but once he believes that someone actually loves him enough to want to save him, his cooperation in what would appear to be an impossible weight-loss effort starts to resemble dogged heroism. Fletcher is introduced as a near-caricature of uptightness and emotional rigidity — even his body type, with its lean angularity, is “censorious” and “chiding” to Pandora — but he later comes to seem genuinely (if grudgingly) angelic as he struggles through his year of living wifelessly. As for Pandora herself, well, she can come across as self-sacrificing one moment and more than a tad dishonest about her own motives the next.

Such dishonesty, of course, is often the calling card of an unreliable narrator. Whether Pandora could actually be in the same highly suspect league as some of Vladi­mir Nabokov’s or Kazuo Ishiguro’s self-interested yarn-spinners doesn’t even emerge as a question in “Big Brother” until it has already been answered. And at that point, once we do get everything sorted out factually, this novel about a brother, his keeper and a radical diet rooted in familial love takes on a strikingly different quality. Regret suddenly begins to wick its way backward through the fabric of the entire story, revealing its concealed moral:There are no tales as outlandish — and no alibis as unbelievable — as the ones we tell ourselves to help us bear the crushing weight of our own sadness.

Turrentine, a frequent reviewer for Book World, is an editor at OnEarth magazine.