Look it from the brother's point of view. Daniel worked hard to launch his career, and then to develop from Craig David-style garage crooner to middle-of-the-road housewives' favourite, before being in a serious car crash in New Zealand. Imagine his surprise, then, at little sis's sudden and seemingly effortless rise to fame. If anything, one of the biggest stars of the past year is being magnanimous in letting Daniel perform with her. Surely there's reason for some antagonism in all that. Yet ask the pair about rivalry, and they talk with real warmth about each other's success and talent.
It's a far cry from pop's original sibling duo, The Everly Brothers. From their rise to fame in the Fifties, this clean-cut duo looked like butter wouldn't melt. It was only when their career stalled that the cracks began to show. By the Seventies, the pair were touring supper clubs and amusement parks, and this relentless, dispiriting schedule led to amphetamine addiction and emotional turmoil.
Don suffered a nervous breakdown, but it was Phil who, on stage in Hollywood in 1973, smashed his guitar and stormed off stage. The duo didn't speak for 10 years, and next met at their father's funeral. They are now performing together again, and don't speak of the rift. It's a mystery to their fans, and the reasons behind it may even be unclear to the brothers themselves.
For the clinical psychologist Linda Blair, though, the patterns are clear. A noted figure in her field of child behaviour, Blair studied children at Harvard, before working with chimpanzees (yes, we will come to Oasis) at Cambridge.
"Sibling rivalry is ingrained, because we need to compete for our parents' attention," she says. "We call this relationship a prototype, because it sets out how you resolve conflicts. But when you get brothers and sisters in music, or business, it's a double dose, because you're faced with the person you learnt those lessons from, so you go back to behaving as you did when you were a child."
Not that Ray and Dave Davies had to compete for attention. They were brought up by older sisters in separate houses. Ray has explained: "Brothers and sisters who live in the same house learn to live with each other's space. We didn't do that early on."
So, as founder members of The Kinks, they became rock's most infamous Cain and Abel. Dave felt he didn't get enough recognition for devising their sound, while Ray fumed as his more handsome brother got more attention from the teen press.
This was apparent in the drunken tantrums, violence and no-shows that undermined their live appearances. In Cardiff in 1965, Dave kicked over the drummer Mike Avory's kit, causing Avory to attack the guitarist with a cymbal or drum pedal. Legend has it that Ray played on while his unconscious brother, blood pouring from his head, was rushed to hospital. Nowadays, fan websites devoted to The Kinks record the siblings meeting for dinner as if reporting some kind of Cold War detente.
Thus the template was set for fraternal competition in rock, with only the toughest surviving. In the Seventies, the Bee Gees arrived at business meetings with their wives and refused to talk to each other. Few people remember Mark Knopfler's brother, David. He left the band after Dire Straits' second album Communique in the face of Mark's complaints that he did not practise enough.
For the Finn brothers, the key is to ration their collaborations. After their band Split Enz broke up in 1985, Neil formed Crowded House; Tim came back in for 1991's monumental Woodface - which was fine until, two hours before their first Glasgow gig, the brothers had a bust-up and Tim flew back to New Zeal-and. They made up in time for 1995's Finn, but that was their last collaboration until last year's Everyone Is Here.
Letting off steam has never been a problem for Oasis's Gallaghers. Many journalists caught their rants on tape, but none so spectacular as the single "Fierce Panda" in 1995, which has Noel and Liam spitting abuse for 14 minutes.
Relationships between sisters are harder to gauge, and groups containing female family members appear to get on better - witness The Corrs. Girls, though, are more likely to go solo, and Kylie and Dannii Minogue have never been moved to bring their sisterly love to the stage.
As a brother-sister combination, Natasha and Daniel are different. Such pairings are more likely to get on. After The Osmonds were torn apart by fraternal rivalries, heartthrob Donny forged a productive partnership with Marie that took in a TV show as well as their duets.
But suspicion hangs over The Carpenters, an embodiment of family values that imploded spectacularly. Karen's death by a heart attack was due to long-term anorexia nervosa, while Richard became addicted to prescription drugs. There is, though, no evidence that their troubles were due to family tensions. Richard's problem was common in the Seventies, while Karen's can be traced back to snide early reviews; one referred to her as "Richard's chubby sister". In fact, The Carpenters had worked their way through various bands, only to find that the best way to reach their potential was to work together.
Linda Blair believes that siblings of different genders get on better because as children they don't compete in the same areas. "There are still different expectations for boys and girls. In fact, they are seen as complementary. We follow the tradition that men and women make a whole: girls are diplomatic, boys aggressive; girls are helpmates, boys are foragers."
Of course, not all brother-sister combos get on. LaToya Jackson is an outspoken critic of Michael, and Janet once complained that she had not spoken to her brother for two years. And they seemed such a normal family.
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