Many parents wish they could freeze a moment, preserve their children just as they are, but few have Thomas Gainsborough’s miraculous skill to achieve it. He painted his daughters Mary and Margaret – Molly and Peggy – with adoration, pride and sometimes anxiety, capturing, as Peggy, the narrator of The Painter’s Daughters puts it, a “closeness so thick you can feel it, our gaze always steady”.

It’s ambitious to draw fiction from such beloved and well-known portraits, but Howes’s fictionalised Molly and Peggy don’t pale in comparison with their painted counterparts. As children they are buoyant, funny and just as devoted to one another as they seem on the canvas: Peggy’s interior world is beguiling and convincing.

She regards their painted “twins” with antipathy. “The inside and the out must sit aligned,” their mother remarks, matching her too-flashy dress to her claimed noble blood, and yet the Gainsborough girls cannot live up to their portraits: they long to “paint [themselves] good” but under the surface things are bad and getting worse. Molly suffers from frightening episodes during which her eyes become “two blank stones in her face”; she talks nonsense, acts unpredictably. The Gainsborough parents choose to overlook her affliction, and the children commit to concealing it as best they can. Peggy becomes her sister’s keeper.

The girls’ progression from cheerful grubby freedom in the fields outside Ipswich to a life where they “look out of the window and sew interminably” in Bath is wrenchingly sad. In the city of pleasure, where Gainsborough paints society portraits and his daughters must present a good face, Peggy’s sense of responsibility for her sister swells. Her helpless anxiety, erupting into violence, is sensitively handled – so too is the cycle of starving, bingeing and purging she falls into. It’s unfortunate that the cause of Peggy’s distress – Molly’s mental decline – is less convincing.

It’s unclear what is wrong: absence seizures? Dissociation? Whatever the episodes are, they’re brief and narratively convenient, and Peggy can snap Molly out of them with violence. It’s paint-by-numbers Georgian lunacy rather than a clear clinical presentation. The author’s note posits that the real Molly Gainsborough lived with porphyria, the genetic condition alleged to have caused King George III’s madness and famed purple urine, but this doesn’t really fit either. Porphyria is a complex disease of the nervous system and the skin; Molly exhibits only the headline symptoms shared by the king.

The attribution of George III’s illness to a physical rather than mental disorder was first suggested in the 1960s and was explicitly seen as relieving the royals of “the taint of madness”. Nowadays he’s more usually regarded as having lived with bipolar disorder. Does this matter in the context of fiction? Howes portrays Molly’s anguish – and her family’s terror – incredibly well, after all. The heart is there.

However, in a novel so concerned with appearances and concealment, image control and damage limitation, it’s strange to settle on a face-saving, debunked diagnosis. It ties into a subplot set about 40 years earlier, which while interesting feels superfluous: this assured novel needed no additional spicing up. It might have been better to excise this and focus on doing justice to one of the conditions Molly Gainsborough is more often suggested as having, such as depression or early-onset dementia. Howes writes so knowledgeably on everything from Georgian pigments to the tensions of sisterly love, it would be well within her authorial powers to accurately and responsibly portray the devastating effects such an illness might have on an 18th-century patient and their family.

Still, the novel shines when it comes to the spooky duality of portraits, of sisterhood, of private and public faces. “I am not you and you are not me,” Molly cries in frustration; and yet, says Peggy, “I can’t begin to tell where she ends and I begin.” The girls are, like their tempestuous parents, “a pair of scales […] when he goes up, she goes down, and it is very hard to balance them”. They aren’t even the only Marys or Margarets: Molly is named for the Gainsboroughs’ first lost child; Peggy is Margaret like her mother and grandmother. These constant facsimiles, twins, doubles, merge into one another, and nobody gets an identity all their own except perhaps Gainsborough himself.

“I am a trompe l’oeil,” says Peggy as her father paints her. She understands that to be moved by a portrait is not to be moved by the real girls, who are not there, but by the talent of their father for making it seem as if they are. The portraits are not simply expressions of fatherly love but an advert for his talents. Gainsborough is always present, “[making] himself visible when it is you, and you alone, who fill[s] the canvas”. The Painter’s Daughters is convincing, engaging, transporting, once again presenting a Molly and Peggy as if they really lived and breathed, and yet our gaze is always directed by the author.

• The Painter’s Daughters is published by Phoenix (£20). To support the Guardian and the Observer buy a copy at guardianbookshop.com. Delivery charges may apply.