Whatever eldest daughter syndrome is, Ollie Shred doesn’t have it. According to the dubious science of birth order and the recent internet, big sisters are the disproportionate load bearers of the family: responsible, organized, high-achieving, resentfully overburdened. According to Amy Shred, her big sister is reckless, self-absorbed, volatile, manipulative. A danger to herself and others.

Told from Amy’s wise, wisecracking and tremblingly self-conscious perspective, “Shred Sisters” is a bildungsroman overcast with the thick and inconstant cloud of mental illness. The story’s arc extends from Amy’s lonely, bullied childhood in 1970s suburban Connecticut, through her college years at Columbia, a false start as a biologist and her gradual success as a book editor in New York City. There she grows into herself, as it were — “at last my reticence was interpreted as allure” — navigating in her early adulthood a fledgling sexuality, meaningful friendships, betrayals, divorce.

The Lila to Amy’s Lenù, Ollie zips in and out of this otherwise conventional story like an erratic hummingbird, sucking the nectar from her loved ones’ palms and then gone as fast as she came.

Lots of ambitious books announce themselves; this one doesn’t need to. The first novel by Betsy Lerner — a veteran literary agent and editor who has written three previous books of nonfiction — it forgoes all fanfare and conceit as it refines a 20-year coming-of-age into an elegant thread of taut, perfectly paced milestones. The prose is controlled, but neither virtuosic nor spare; the plot, enticing but neither Dickensian nor minimalist. Decidedly un-trendy, crescendo-less and restrained, this tragicomic family saga is a Bach prelude to the Rachmaninoff of a writer like Jonathan Franzen.

The restraint has a purpose: We feel Amy Shred clinging to her calm for dear life. Ollie (short for Olivia) begins disappearing from the house as a teenager, and Lerner forecasts from the beginning how much worse it’s going to get. “With every disappearance, my parents recalibrated the idea of normal as it shifted beneath us,” Amy says. But despite the chaos that accompanies her sister at home — the glass window shattered with her body; the tantrum on a family road trip that very nearly causes a multicar pileup — somehow “Ollie’s absence was more suffocating than her presence.”

By adulthood, Ollie’s all but estranged, in and out of psychiatric hospitals, making her way as a sex worker and making off with their father’s $70,000 investment in a bakery she has promised and failed to open. At 18 Ollie ends up in “the Place,” the never-named institution she’s sent to after stealing a fur coat and $60,000 worth of silver from a classmate’s parents. (The Shred sisters’ blunt, boomer mother, bless her ice-cold heart: “All this therapy was just a way for privileged kids to avoid legal consequences for their bad actions.”) Amy shows up for a family session and brings a card for Ollie that says only “This sucks.” Ollie’s slivers of pathos hit hard: She calls Amy crying and says, “You’re the only one telling the truth.”

Telling the truth, of course, is our narrator’s solemn duty, her exalted role: the observer, the over-thinker, the scribe. She positions herself not as a sidekick but “on the sideline” of Ollie’s adventures. At a rest stop on that road trip, as Ollie is cawing at a murder of crows, Amy sees another family flee the scene and thinks, “The curtains had parted, and our family was yet again in the middle of a play we hoped no one else could see.”

But in Amy’s own play she is naturally the star, Ollie relegated to the supporting role. In a subtle way, we sense the sisters have more in common than Amy lets on. Both are whip-smart and beautiful; both reserve a biting sarcasm for anyone deemed insufficiently cool.

Implicit in Amy’s demure pose is a self-regard equal in magnitude to her self-loathing; she doesn’t just resent the world for deeming Ollie’s star brighter than hers, she also disagrees that it is. “What about me?” she asks her parents in her mind only — but of course, we hear her. As intoxicating as Ollie’s appearances tend to be, it’s Amy’s experience of them — as well as of the ripples in her own life, from the freeloading aspiring actor who breaks her heart to the handsome lawyer husband who tries to fold her into his “shallow” Princeton friend group — that elicits our deepest reserves of pathos.

The term “bipolar” does not appear until late in the book, “manic depressive” only once, well into Ollie’s adulthood. Lerner’s resistance to the terminology is perhaps a reflection of the Shreds’ time and place, the still-nascent popular understandings of the variegations of mood and its disorders; but it also effectively consigns neat diagnostic categories to the inscrutable realm of late-20th-century psychiatry. No place for that here: This is a book about what happens on the ground. Sifting through Ollie’s old records from the Place years later, Amy approaches her sister’s mind from a more humanistic perspective, her care simply crushing:

I wondered what Ollie’s life trajectory might have been had she followed Dr. Lucie’s recommendation, stayed on the medication long enough for it to be effective, level out in her bloodstream, stabilize her moods. Or would it have been another ghost we were chasing? And who’s to say Ollie didn’t find her life satisfying? You could say she lived on her own terms, or you could dismiss her as mentally ill, the woman who continues to dance after the floor clears and the lights go down, listening to music only she can hear.

I suspect that there is no “right” way to handle a loved one’s mental illness, and Amy, her mother and her father all differ in their approaches. Call it enabling, but none of them give up on Ollie. Her father dispels the haters with a Shred-style shrug: “What am I supposed to do, disable her?”