A new film sheds light on how sibling rivalry can turn your closest friend into your most hated foe, writes Deirdre Reynolds

She's supposed to be your closest friend and confidante. Yet in reality, she's often the biggest bitch you've ever met in your life.

The latest film to hit cinemas here shows how the harmless hair-pulling that goes on between young sisters across the world can degenerate into full-blown battle.

Based on the best-selling novel by Philippa Gregory, The Other Boleyn Girl charts one of the most explosive cases of sibling rivalry in history. It tells the tale of aristocratic sisters Anne and Mary Boleyn, who went to war over sex-mad monarch King Henry VIII in the 16th century. And the true story, starring Scarlett Johansson and Natalie Portman, reveals how women who are flung together by birth can wind up becoming bitter rivals rather than bosom buddies.

For sisters flocking to see the flick together over the weekend, there may have been a few moments of uncomfortable silence. And while pinching your clothes, helping themselves to the last of your favourite perfume, or eyeing up your man might not be on the same level as tearing the heart of the Tudor palace in two, they underline the complex blend of backstabbing and loyalty that define sisterhood, according to psychologist Dorothy Rowe.

Party girl pals/catfight combatants Paris Hilton and Nicole Ritchie may have propelled the term 'frenemy' to the urban dictionary, but for its origins look no further than your own family.

Dr Rowe, who wrote My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bonds, reckons most women both love and loathe their skin and blister at various times throughout their life.

"The reason I came up with the title was to try and encapsulate that deeply ambivalent relationship between most siblings," she added.

"Up to now, psychologists and therapists have been looking at the relationship between the child and mother; but while doing that, they've steadfastly ignored the relationship between siblings.

"Yet we all know how important siblings are -- they're the people we have the longest relationship with and it doesn't matter how old you get, the connection is always there. I decided to look at how siblings form an attachment to one another and explain the dynamic that develops."

And although the study looks at both brothers and sisters, Dr Rowe says things usually get more complicated between the girls.

"Siblings can demolish us," she added. "You know those frequent occurrences, when your sister says something which to an outsider might seem quite banal, but you've been struck through the heart with an arrow by it.

"You know the other so well. They know the things that upset you most and those that give you deepest pleasure.

"The book is about both brothers and sisters, but I think women talk about it more. A lot of men aren't troubled by not keeping up with relatives or they leave it to their wives."

Just because you're related, there's no guarantee you're going to get on, Dr Rowe explained. But whether you bond or bicker, your filial feelings could colour your relationships for the rest of your life.

"If you look at conflicts around the world, the ones that are hardest to resolve involve some kind of family relationship," she said. "The war between the Israelis and Palestinians is effectively a family squabble. Likewise, in Northern Ireland, Catholics and Protestants fought throughout the years but they also bedded each other. There's no genetic difference between these groups and they row like siblings."

She added: "Siblings are our first peer group, so even when we go to school and work we often see others in terms of our relationship with them -- sometimes causing us to make really bad mistakes. For instance, if you've got a female boss, you might immediately assume she's going to be as nasty to you or look after you the way your sister did."

And sibling rivalry starts the second an interloper arrives in the house to steal the affections of mum and dad. Whether you're the halo-wearing first born, overshadowed middle-child or attention-seeking youngest, your placement in the clan can determine whether you enjoy girly get-togethers with your sis, or barely stand the sight of each other in the years to come, says Rowe.

"When you're an only child, you're in this nice situation of being treated like a prince or princess by your parents or grandparents. Suddenly, this thing turns up and you're displaced, and it can cause tremendous trauma for the child -- even when the parents deal with it well.

"I remember one young woman who found out she was pregnant with her second child and did all the things you're supposed to, like telling her daughter about the baby in mummy's tummy, involving her in shopping for the baby and giving her a present the day the baby was born. She thought everything was fine until her daughter turned around and said, 'Couldn't you have kept that thing inside you for another while?'"

She added: "The older sibling is a trailblazer, while the younger ones test the limits set by them. Within families, there's usually a 'good' sibling and a 'bad' one and it can be difficult to break out of that mould."

No matter how many years pass, it can be impossible to view your sister as anyone other that the girl who mutilated your beloved teddy bear, snitched to your parents when you smoked pot in college, or beat you down the aisle, Dr Rowe revealed: "Most people I spoke to say that when they're with their siblings, they feel like a swan -- calm on the surface but paddling underneath."

And society may frown on it, but don't be afraid to give a malicious sis the flick.

The author, who confessed to having a tumultuous relationship with her own big sister, said: "Women can feel guilty if they haven't fulfilled what is expected of them by society and so they maintain a relationship with a difficult sibling and do things for them they wouldn't for a troublesome friend.

"We get caught up in this thing of blood being thicker than water. But I think you've got to work out the cost of keeping in touch with someone who really isn't on your side. If she was a friend who was consistently horrible, you'd drop her."

But provided she isn't trying to get her claws into your hubbie, a la Anne Boleyn, it is possible to make her a mate -- and not just a relation.

And in spite of the contradictory concoction of devotion, rage, envy, competitiveness and loyalty they inspire, sisters can be the best friend a girl will ever have.

Dr Rowe joked: "I think sisters are more likely to row over who's got the better job nowadays, than over a man like the Boleyns! But once there isn't an insurmountable level of resentment there, it's possible to have a great relationship and look after each other in that way that only sisters do."

SISTER ACT: Celebrity siblings

Elle and Mimi Macpherson: What do you do when you live in the shadow of a big sister known simply as 'The Body'? Make a sex tape, silly.

Although already a celeb in Oz, supermodel Elle's baby sis hit international headlines in the late Nineties when she starred in an X-rated home video with then boyfriend Michael Hellwig.

It's not the only attempt the presenter has made to distance herself from her wholesome sibling.

Black sheep Mimi, who has a string of drink-driving convictions to her name, changed her name when she set up her whale watching business Down Under.

But the business took off when she reverted to her famous family surname.

Ashlee and Jessica Simpson: Erstwhile raven-haired rocker Ashlee appears to be morphing into busty big sister Jessica by the day. And psychologists could probably have a field day with the reasons why she's coming over all Single White Female on her own sister.

Manager dad Joe moved the family to LA in a bid to boost bombshell Jessica's profile -- at the expense of her younger sister's budding ballet career back home.

Crooner Jessica shot to fame on MTV reality show Newlyweds after saying 'I do' to pop hunk Nick Lachey.

But wannabe Ashlee became the butt of showbiz jokes, especially after a reddening lip-syncing incident on Saturday Night Live.

These days, though, Ashlee appears to be the paparazzi's favourite since going public on her relationship with Fall Out Boy bassist Pete Wentz.

Kylie and Dannii Minogue: In 1980s Melbourne, two young sisters were on the path to stardom.

However, one of the starry-eyed pair could only dream of duplicating the success of the other, who was already a regular on top TV variety show Young Talent Time.

In those days, it was younger sibling Dannii who stole the show. But after landing a part in Neighbours, the scales tipped permanently in favour of petite pop queen Kylie. Both sisters have embarked on music, acting and fashion careers, with Kylie triumphing each time.

Despite the reversal of fortune, though, Dannii denies that there's any sibling rivalry seething beneath the mega-watt smiles: "We were raised in a family of love and support and that has not changed."

The Corrs: While most siblings are busy fighting over the remote control, Dundalk clan The Corrs lived in perfect harmony -- literally.

Trio of sisters Caroline, Sharon and Andrea, and brother Jim, spent their teenage years practicing music in his bedroom. And they finally hit the big time performing at the Summer Olympics in 1996. But despite being surrounded by sizzling siblings, it was ethereal beauty Andrea who hogged the limelight.

And as the rest of the family swap touring for teething, 33-year-old Andrea, who's dating multi-millionaire Brett Desmond, has a monopoly on the media's attention.

'We now laugh about the awful things that we did to each other'

Sisters Anne-Marie, 28, and Patricia McKenna, 25, from Moynalty, Co Meath, reveal how they turned a troubled relationship around to become best friends as well as family.

Anne-Marie says: "Patricia and I were not at all close when we were young. As there are five girls in the family, you had to choose to team up with the strong ones to survive; Patricia was not one of the strong ones -- I never chose her. She was so gullible and did almost anything we asked her to do and she always got in trouble for it.

"However, when I finished college and got a job, I was living at home and realised that I was lost; Trish took me under her wing. We went out together every other weekend and had a great laugh. We're still really close and I even asked her to be my bridesmaid! We now laugh about the manic childhood that we had and the awful things that we did to each other."

Patricia says: "Anne-Marie is three years older than me. When we were growing up, there were five of us around the same age so I stuck with whichever gang suited the situation! Anne-Marie and I went to the same primary and secondary school, but I can honestly say that we weren't friends until I was about 20. She was always studying, whereas I preferred to daydream. When we both moved to Dublin for college we lived together for a year, but never really spoke.

"I suppose we didn't really have that much in common. It all changed in 2004; she broke up with her boyfriend when I was in my final year of college. We started doing things together -- socialising, going out drinking -- and became good friends. We're still very close. I think she mellowed a little and I started taking life a bit more seriously."

- Deirdre Reynolds