When it comes to genes life's a lottery . . .

As Abi would the first to know. She has spent her life in the shadow of her stunningly beautiful, glamorous older sister Cleo.

Headhunted as model when she was sixteen, Cleo has been all but lost to Abi for the last twenty years, with only a fleeting visit or brief email to connect them. So when Abi is invited to spend the summer in Cleo's large London home with her sister's perfect family, she can't bring herself to say no. Despite serious misgivings. Maybe Cleo is finally as keen as Abi to regain the closeness they shared in their youth?

But Abi is in for a shock. Soon she is left caring for her two young, bored and very spoilt nieces and handsome, unhappy brother-in-law - while Cleo plainly has other things on her mind. As Abi moves into her sister's life, a cuckoo in the nest, she wrestles with uncomfortable feelings.

Could having beauty, wealth and fame lead to more unhappiness than not having them? Who in the family really is the ugly sister?

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Genetics is a strange science. It’s imprecise. A jumble of random combinations that make a unique whole. It’s basically a lottery. So you can inherit the beautiful turned-up nose of your mother, but the fact that it’s so big it takes up half your face from your paternal grandfather. Or your long legs from one side, but their tree-trunk-like appearance from the opposite. Or sometimes, cruelly, one sibling can inherit all the available beauty, leaving nothing for the other except the cast-off bits that didn’t quite make the grade. It’s as if there’s only so much good stuff to go round. Or at least that’s how it has always seemed to Abi. But then you could say her perspective is a little warped.

It’s a subject that’s close to her heart. One that pre-occupies her from time to time. The unfairness of it. The way that your inherited characteristics can deter-mine the course of your whole life. Or, to be more exact, the way someone who shares more than their fair share of DNA with you can have such a different experience of the world simply because of the few small dissimilarities in the chain.

She knows too that the universe always appears to be a different place to siblings just by virtue of their place in line. The oldest – used to being the entire focus of their parents’ adoration for as many years as they remain alone – entitled and imperious, convinced the planets revolve around them. The younger destined to grow up in the shadows, eager to please, aware that their very arrival has destroyed the idyll of the indulged first born and so prone to apology and self-sabotage.

In a large family you can get away from the endless comparisons or, at least, you can manipulate them to make yourself feel good. If measuring yourself against one of your siblings is making you feel bad, pick another. There’s always someone you can feel superior to. When there are only two of you, though, there’s no escape. Your every feature, every ability, every quality is held up for scrutiny and direct comparison. And the sad truth is that some people are simply handed a bigger deck, a shinier, sharper set of tools. Some people, as the saying goes, have all the luck.

She’s contemplating this – for the millionth time, or so it seems to her – as she stands on the doorstep of her sister’s palatial Primrose Hill home, gazing up at the sheer magnificence of the architecture, suitcases at her feet, wondering why no one is answering the door. Actually it’s an exaggeration to say she has suitcases; she hasn’t been on a proper holiday for years – why would she even own a suitcase? What she has is a large green nylon rucksack, an oversized Debenhams carrier bag stuffed full of last-minute bits and pieces, and a smaller Nicolas one containing a bottle of cheap (ish) champagne by way of a thank-you gift. Looking around the upmarket neighbourhood, she has no doubt that house prices in the area must be dropping by the minute the longer she stands there.

In fact, for the past month or so, ever since the email pinged into her inbox reminding her that she had a sister in the first place, Abi has thought about little else. It wasn’t so much the fact that she received a communication from Caroline that set her off. Well, in part it was: Caroline doesn’t often get in touch outside of birthdays and Christmases and then only in a sort of formal and disinterested way. A duty call. The real surprise was what was in the email. The fact that it was an invitation to go and stay. And not just for a night either, for the whole two months of the summer holidays. Abi can’t remember the last time she and Caroline spent eight days together let alone eight weeks. Or seven for that matter. Or even six. Three, maybe, and that would have been a few years ago. A strained Christmas at their mum and dad’s, probably.

Abi had to read the email four times to make sure that was really what it said:

It’s been so long since we’ve spent any time together. I really regret that we’ve grown so far apart, and I miss having you in my life. Go on. You never know, it might be fun!

She had called Phoebe away from the TV to come and verify that she hadn’t lost her mind.

‘What do you make of this?’

Phoebe read it quickly, bending over Abi’s computer, holding her too-long dark-brown fringe away from her eyes as she did so.

‘Wow,’ she said, ‘are you going to go?’ ‘I don’t know. What do you think?’

‘She’s right – it might be fun. After all, I’m not going to be here,’ Phoebe said hesitantly, and Abi instantly felt guilty about being the only family Phoebe had that she really knew.

‘Why do you think she’s invited me?’ Abi asked. Her daughter, all of eighteen years old, usually had an opinion on everything.

Phoebe shrugged. ‘She’s your sister.’

Phoebe would love to have a sister, so in her mind it’s straightforward: if you have a sister, they must want to spend time with you – that’s how it works. She’s young – she’ll learn.

Abi scanned the email again. ‘I suppose it’s true. It would save me having to rent a flat. And if you’re not going to be around anyway . . .’

Abi, painfully aware that her only child was about to leave home, had recently sold the small cottage, two streets away from the seafront, where Phoebe had grown up. She was downsizing, moving into a shoebox-sized flat with views across the town from the tiny balcony, and a minuscule spare room with a sidelong glimpse of the sea that would be Phoebe’s whenever she came home for a visit. She had got her timings all wrong, though, and was going to have to vacate the cottage weeks before she could take possession of the new place. She had been planning to rent in the interim, something she could ill afford to do, and it was when she had told Caroline this in a routinely small-talky email, of the kind Abi regularly sent but to which she seldom received a response, that the invitation had come. Now she could put all her belongings into storage at a fraction of the cost and spend the summer in London with her only sibling. It was a terrifying thought, but it was a solution. More than that – much more if she were being honest – there was the hint of a reconnection with her sister. For the first time since, well, for as long as Abi could remember really, Caroline seemed to be reaching out to her and, if that was the case, she didn’t see how she could pass the opportunity up.

‘Maybe.’

‘What have you got to lose?’

‘I suppose it might be good to spend some time with Caroline. I never see her these days.’

‘You mean Cleo,’ Phoebe said. ‘Don’t call her Caroline, whatever you do.’

‘Yes,’ Abi said quickly. ‘Of course I do. I mean Cleo.’