Sisters. Sisters. There were never such devoted sisters. That's how the Irving Berlin song goes and, when I was little, I remember sitting on the sofa, watching White Christmas, and gazing up adoringly at my older sister, Nicola, and thinking that that song was written about us. Weren't we devoted? Of course we were! Didn't we share everything? Absolutely. Only three years apart in age and as close as close could be until... well, until I cajoled her into letting me have her favourite dolly, Michelle for the night and one of the doll's eyes fell out. Nicola didn't speak to me for weeks after that. Or maybe our relationship began to crumble when she was allowed a pony and I wasn't and I harboured such a grudge that, years later, I took her favourite lipstick and smeared it all over the walls of her bedroom. Or did the problems begin when she had a boyfriend and I didn't and I used to kind of flirt with him (poor bloke) in my own childish way?
Oh, sisters! What a tangled web we weave. Forget mothers and sons, fathers and daughters, it's the relationship between sisters that is fascinating. Sisters can change the world. When we are good, united, pulling together, we are an unstoppable force. Look at the Middleton sisters. Would they, on their own, have made the Time Top 100 most influential women in the world? Probably not. It's the combination of them, with their conjoined serious/flirty double-act thing, and their irrepressible rise to the top of society's echelons, that makes them quite so powerful. They weren't bred for it. They have willed it.
And yet, and yet ... anyone who has a sister will tell you how fraught the relationship is. When we are in opposition to each other, we can bring houses down with the venom of our dislike. For it is just so complicated. Sister relationships are bound by two utterly opposing factors; you love them and you hate them. I have felt more passion, more fear, loathing, love, loyalty for my sister than just about anyone else on the planet. She can drive me utterly insane – why does she do the crazy things she does? Then melt my heart with the warmth of her human kindness.
On the one hand, sisters are usually, on some level, utterly devoted. This is the expectation of sisterhood – you are both women! You have to pull together! On the other hand, we are all too close to each other for comfort.
Sisters share a very strong bond – they belong to the same family and are of the female gender. This, in itself, provokes a very strong glue. How can brothers truly understand their sisters and vice versa? Sisters go through the same things together – adolescence, first boyfriends, choosing bras, sharing experiences of pretty much everything. Some sisters seem almost programmed to react to a similar set of circumstances in exactly the same way.
My sister and I appear to be like chalk and cheese. She hasn't had a career. I have. She is an "earth mother", I'm not. She's really nice to sad, lonely hangers-on. I am impatient and dismissive. She gives time to everyone, saves lame ladybirds, rears tiny chicks by hand. I march onwards and upwards, inadvertently crushing everything in my path.
But, when you look at our lives more closely, it is incredible how similar we are. We have both had lots of children. We both love the country. Our love lives have virtually mirrored each other's. This means that, regardless of all our ups and downs – and there have been many – we have a real, true, innate understanding of each other. And yet we have crashed and burnt so many times. We spent years not talking to each other – my mother stuck between the two of us, desperately trying to keep the peace as my sister and I have lobbed hurtful, painful insults over a metaphorical wall.
Sometimes I have been stratospherically insulting to her. She has bitten my head off at any given opportunity and yet, if anyone is rude about my sister, I'll leap to her defence. I will bristle on her behalf, defend her to the hilt, take up my cudgel and wade on in. I have shouted at her, sworn at her, and yet I've cried on her, wept on her like a baby, asked her to look after me, listen to me, help me. When the chips are down, we are there for each other.
We are the best of friends, the worst of enemies. There is no one on this planet who drives me quite as mad as she does, and yet... she's the first person I ring when I am in trouble.
"Of course the sister relationship is fraught," says family therapist Susan Swire. "Look at all that competition! There's the whole problem of 'who's the prettiest' and 'who's the most talented'.
"Then there is the inevitable tension around boyfriends. When girls are younger, they are very competitive about boyfriends and then, as they get older, they get competitive about everything. We think of our sisters as mirrors, and, sometimes, they don't show us at our best, and then that all gets turned into an intense form of rivalry."
She's right – there are countless examples of sisters who indulge in a bit of healthy sibling rivalry, especially when it comes to the attractiveness stakes; Paris and Nicky Hilton are always posing together and yet, by their very natures, Paris is known more than Nicky because she is the most flamboyant. Vogue fashionista and author Plum Sykes is more widely known than her twin, Lucy, even though they both worked in the fashion business for years. Kim Kardashian is more famous than her sisters, Kourtney and Khloe.
This always seems to be the way. One sister rises higher than the other, the more flamboyant garnering more attention than the sober-suited one, the good versus the bad, each sister being the flipside of the other one, a female version of Jacob and Esau.
Take Santa and Tara Palmer-Tomkinson. Blonde, beautiful, honey-skinned Santa is a long-time married (to historical writer Simon Sebag Montefiore) mother and novelist. Tara is naughty, outspoken, formerly wild, vulnerable, and much loved by the nation.
Look at the Minogues. What do you do if you are Dannii? It's impossible to outdo Kylie who is deemed to be a total sweetheart by everyone. She was Charlene in Neighbours! Dannii didn't stand a chance. But maybe it's not that simple. Some sisters just don't get on. Novelists and sisters A S Byatt and Margaret Drabble haven't spoken in years, and refuse to read each other's books.
It also doesn't help when one sister is far more successful in their field than the other. Kristin Scott Thomas is renowned for being a great actress. Her sister Serena Scott Thomas, a former Bond girl, is almost forgotten. Jackie Kennedy Onassis outshone her sister Lee Radziwill, even though the latter was a princess.
My own sister and I have actually flipped roles. As a child, she was more serious, more controlling, more disapproving than she is now. I was probably more rebellious, cheeky, naughty, fun, and now, while I still have a touch of the rebel, my sister has become positively carefree. That serious little girl with the pursed lips has disappeared, to be replaced with something far more joyous and vibrant.
I'm the disapproving one now, always furrowing my brow, shaking my head, thinking "Oh no!" every time she rings me up to tell me a tale she finds hilarious and I find terrifying. When she told me she was pregnant with her seventh child, I almost dropped the phone in horror. We didn't speak for ages after that. But once she had her beautiful baby Jack, I felt truly mean for being so disapproving of something, someone, who is such a big bundle of joy.
Who was I to judge? Well, I am her sister. That's the point of me, her, both of us. Sisters, sisters. There were never such devoted sisters.