Sibling rivalry is a story as old as humankind: Cain and Abel started it, Romulus and Remus kept it going, and we've not been able to shake it since.
But what happens when two siblings lock horns? How do parents cope when their children enter into the same career? Or when one child seriously outshines another? Is it possible to keep rivalries friendly?
Famous sibling rivalries
The sporting world is full of athletic brothers and sisters, competing in similar fields. The Williams sisters are one of the most high profile sporting siblings around. They make no secret of their desire to win, and yet somehow the two remain firm friends, playing doubles together and cheering each other on.
Perhaps it's easier for them because they have both dominated the sport at one time or another. But how about the Murray brothers – Jamie and Andy – who have had very different careers in tennis, with younger brother Andy outshining his older sibling?
The pair have disagreed publicly in the past, but their mother Judy is adamant that they are very close, telling the Guardian, "they can say things to one another that they probably wouldn't say to anyone else. But there's no jealousy or rivalry."
Jamie is certainly more laid back than his younger brother, and despite not having such an illustrious singles career or healthy bank account, Jamie is the only one of the two to have a Wimbledon title to his name as winner of the mixed doubles in 2007.
Some siblings have made their career out of a public rivalry. Noel and Liam Gallagher took over from Ray and Dave Davies of the Kinks as music's most warring brothers. Meanwhile, Dannii and Kylie Minogue have managed to stay united in spite of the press. Dannii has been quoted saying uncharitable things about Kylie's feelings when Dannii fell pregnant and about her older sister's alleged surgery – things both sisters strenuously deny.
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And while Dannii has lived her whole life in her big sister's shadow (despite finding fame in Australia first), she's finally carved a niche for herself thanks to appearances on Australia's Got Talent and X-Factor
Parents who make it worse
But how do you cope if you have two children who have very different abilities? What happens when one is more charismatic, or successful, or happy than the other?
It’s not just famous people who are affected. When Sally Hughes was 10 her mother took her to one side and said "you must never call your sister stupid. You are cleverer than her and it's not fair of you to make her feel bad about that."
Sally's sister, Rebecca, is two years older and spent her entire school years being told by teachers "you're not like your sister are you?"
"It was weird for me to be compared to a sister who is younger than me," says Rebecca. "I was supposed to be older and wiser, but I just wasn't quite as academic as her -- or as good at sport and music".
But, as Sally explains, "it was frustrating for both of us because Becks wasn't bad at stuff. She definitely found it harder, but she didn’t really try because she knew my parents didn't expect anything from her".
Meanwhile, Sally was growing increasingly frustrated that her hard work was ignored. "It wasn't like I was so brainy I didn't have to work. But if I did well in an exam, my father would just ask why I didn't do better: 'you got 93%? Why didn't you get 94%?' A lot of it was meant jokingly, but I don't think my parents once said 'well done' to me or 'we're really proud'. They thought I'd get a big head and that Becks would feel bad.
Rebecca and Sally are very close, but that sibling rivalry still lingers between them. "We don't really talk about it", says Rebecca, "but I think we both feel annoyed that our parents set such different standards for us -- and that I ended up feeling like the pitiable thick one while she felt like the un-praised brain."
Sally went on to have a fairly high-profile career while Rebecca did what she'd always wanted to do – became a full time mother. "I think it really helped when Becks became a mum. She's brilliant at it. I ring her all the time for advice on raising my own. It's redressed the balance – I'm not bossing her about and our parents aren't comparing us."
"It's certainly nice not to have teachers mistaking me for my little sister any more," laughs Rebecca "it's meant to be the other way round!"
Rebecca and Sally's parents struggled to deal with two very different siblings, but it's not uncommon for parents to find it tough to deal with.
Coping with rivalry
British Olympic athlete Jessica Ennis is very different to her sister Carmel. Their mother, Alison Powell, says, "Jess is the most focused and determined person I’ve ever met – I’m in awe of it. Her sister [Carmel] is more laid back, more loving and giving. Not that Jessica isn’t," she hastens to add, "it's just her determination to be the best is very unique."
She found dealing with two very different temperaments took time to get used to:
"I had to be really careful to make sure they felt equal. Sometimes I think I over-compensated with Carmel, because it’s hard being a younger sister with your older sister doing so much. Then I had to make sure Jess wasn’t feeling left out. It was difficult, making sure both my kids felt loved and supported in different ways.”
Joanne Mallon, author of Toddlers: An Instruction Manual says it’s important to be fair and consistent in how you treat all your children, “Children will naturally have different strengths and talents, and children of different ages will inevitably be treated differently, which to them may translate as unfair.”
But it’s not all doom and gloom: learning to let a sibling have the limelight some times is a good life skill for children, says Joanne, “your child is learning very important interpersonal skills, and also that the world doesn't revolve around them 100 percent of the time”.
She suggests finding things that you can all do together as a family, “play board games, do jigsaws, go for a bike ride or a swim together. Be careful to praise your children for their own individual talents. Notice times when they play well together and praise that.”
So sibling rivalry may be as old as time, but it doesn’t have to stop your children from being the best of friends – even if they do like to go to war from time to time.
At the heart of Team Mum is the video series Raising an Olympian, sponsored by Proctor and Gamble, profiling athletes from across the world, their dedicated efforts to make it to Olympic Games, and the mothers who had tremendous impacts on their lives. Watch the videos on Yahoo! Lifestyle Team Mum.