In Deborah Moggach’s latest novel one of the middle-aged characters observes: “One has to make allowances with the elderly. But he didn’t want to. He wanted his old father back, bristling with questions. He didn’t want to treat his dad like an invalid.”

With The Carer, Moggach explores the topical question of care for the elderly and whose responsibility it is. She infuses what could be a dry subject with her trademark humour and pathos, reshaping a societal dilemma into a family drama, by turns compelling and surprising.

Robert and Phoebe’s father, James, has broken his hip and needs full-time care. Robert lives in Wimbledon, Phoebe in Wiltshire, and “their father’s village lay between the two of them”. In the past, the siblings have argued about which of them should visit at times of crisis: “Robert was nearer, but traffic was diabolical, getting out of London. He also subtly reminded Phoebe that he was a family man, with commitments. In fact, his children had long since left home and all he did was sit in his garden shed trying to write but she didn’t like to point this out, because mentioning his novel wasn’t advisable, dear me no.”

So they employ a live-in carer, Mandy, to look after their father. Mandy is “not beautiful – far from it. An overweight woman in Rosemary West specs, wearing a bobble hat and stripy tights, something vaguely blokey about her.” And yet within weeks she has inveigled her way into their father’s affections: calling him pet names, escorting him on outings he would have previously sneered at and taking an apparently unhealthy interest in his personal paperwork.

Moggach beautifully portrays Phoebe and Robert’s ambivalence towards Mandy: they are scathing about her, but need her. When she begins to behave suspiciously, neither of them dares to corroborate the other for fear of the inevitable consequences: having to fire and replace her, which neither has the time or inclination to do.

Mandy’s intimate care of James is described with visceral acuity, as is Robert’s and Phoebe’s squeamishness. Through this quartet, Moggach astutely conveys the complex emotional and transactional dynamics of outsourcing the care of a loved one.

And yet when the revelation about Mandy’s true motivation is delivered, a third of the way into the story, it is unexpected. Moggach successfully subverts our expectations, taking the novel into unforeseen territory, exploring themes of desire, expectation and the ownership of collective family history.

All of this is rich territory, but Moggach’s nuanced characterisations make the subject matter really come alive. Phoebe is “a single woman, of a certain age, and childless. This suggested an Austen-like obligation to others, but she was buggered if she’d let Robert off the hook.” Robert is married to a TV news presenter, but having been made redundant from his job in the City now worries “that if somebody cut them open they’d be appalled at what they found”. The siblings’ rivalry is vividly conveyed: “skirmishes could still erupt, especially when alcohol was involved”.

This, at its heart, is a novel about family mythologies: about the stories we are told and the stories we allow ourselves to believe in order to protect our perceptions of childhood and parents. As Robert and Phoebe observe when they have begun to uncover the truth: “They knew life was messy – who didn’t, at their age? – but nothing had prepared them for this.”

• The Carer by Deborah Moggach is published by Headline (£16.99). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99