It is our elder sister’s fortieth birthday party and, as always, she is a vision of loveliness. She’s wearing a floor-length Vera Wang gown in white that’s drenched in Swarovski crystals which burst into tiny rainbows whenever they catch the light. It fits like a glove, hugging every inch of her tiny frame. Her auburn curls are piled high and her head is thrown back in uncontained and joyous laughter, showing off the delicious creaminess of her throat and her perfect teeth.

Her husband Richard is tall, tanned and has the rugged-jawed looks of a classic romantic hero. He’s something fabulous in the City, is stonkingly wealthy, is kind to small furry animals, remembers to empty the bins and puts the lid down on the toilet when he’s finished too.

‘What star sign is Chelsea?’ Lauren wants to know.

‘Gemini.’

‘And that means you’re the luckiest bitch in the world, does it?’

‘I think it means you’re a good communicator with a sunny disposition.’ With a tendency to be two-faced and selfish on occasions if you believe in the stars, but I won’t tell my sister that.

Lauren tuts and throws back her drink. ‘What are we?’

‘Scorpio.’

‘And?’

‘We’re the most murdered star sign.’

Another tut. ‘Fabulous.’

This isn’t your usual fortieth birthday party. For mine – next year – I was thinking of having an At Home with a few family and friends, a readymade cake from Asda and some balloons – if I could summon up the necessary energy. Chelsea’s birthday party – like the rest of her life – is exquisite.

We’re at the Dorchester Hotel on London’s Park Lane in the main ballroom, along with about 300 other people. Already I’ve spotted Jodie Kidd, Jamie Oliver and Richard Hammond – just a few of the celebrities that our sister counts among her closest friends.

‘Where did we go wrong?’ Lauren asks, as she slides down on her pretty silver chair. She sounds a little drunk. In the same manner that Gordon Ramsay uses a little bad language. Oh – and he’s here too, somewhere.

I sigh before I answer, ‘I don’t know.’

Needless to say, Lauren and I haven’t gone down the Vera Wang route of haute couture. I have chosen vintage Next – in other words, a dress that’s at least three years old. Not even a good vintage, I feel. My sister has opted for a little number from Coast that will be owned by her credit-card company for the next year or more and, I don’t like to mention it, but there’s some of the raspberry coulis – from our bitter chocolate mousse with raspberry coulis dessert – down the front of it.