BIG BROTHER. By Lionel Shriver. Harper. 373 pages. $27.

Lionel Shriver’s personal life has garnered almost as much attention as her acclaimed novels. Her candid admissions that she only allows herself one meal a day and has shunned motherhood for fear of not loving the child as much as herself, have made her a polarizing public figure.

After the sudden, international success of her 2003 novel “We Need to Talk About Kevin,” a dark story about a teenage shooter and his apathetic mother, Shriver was often asked to defend the darkness of her novels and of her mind. Her response was unapologetic and stern: it was humanity’s fault, not hers.

In her new novel, “Big Brother,” Shriver looks at a problem that is as private as it is public: morbid obesity.

Pandora has always idolized her older brother Edison. He was attractive and musically talented, so no one was surprised when he headed to New York as a teen and became a successful jazz pianist. Meanwhile, Pandora found her own success when she turned a prank gift into a highly lucrative business making customized pull-string dolls. Her family was acquired as suddenly as her professional success when she married Fletcher, a fitness freak with two teenage children. Their marriage, like her husband’s strict diet, is bland.

When Pandora’s estranged brother calls and asks to visit, she knows he is “bound to breathe some life into that vast blank house ... that, since the advent of Fletcher’s mad cycling and cheerless diet, had erred on the grim side.”

She quickly agrees to host her brother, and like the mythical Pandora, lets loose a chaos she can’t anticipate.

When Edison is wheeled into the baggage terminal of the airport, he weighs hundreds more pounds than when they last met. Pulled between social niceties and her own disgust, Pandora at first makes little of having to find a chair that will fit her brother’s new size.

His short visit turns into a longer one, which puts a strain on more than just her husband’s handmade chairs. Forced to choose between her familial and marital bonds, Pandora discovers that “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 from you. ... Ergo, what Edison could ‘reasonably’ expect from me was infinite.”

Picking up any of Shriver’s previous novels guarantees an unflinching and insightful glimpse into a hot-button topic.

With her latest, however, the momentum never gets started. Like her character, the novel seems bloated and in need of parsing. Having admitted that she’s drawn to the perilous territory of the human mind, one would expect to excavate the shadowy corners of these characters; but long passages ramble on, making the characters more clouded than visible.

A twist at the ends adds little to the novel and might even frustrate readers. There is ambivalence in the novel, an emotional blandness that marks Shriver’s style, though in “Big Brother,” it seems to distance the reader from the characters, making it difficult to care about them and their revelations.

Reviewer Summer Mauldin is an editor and writer living in Charleston.