Amy Jenkins: Don't be fooled when siblings talk about being friends

I mostly get on with my brother and sisters and so have never thought much about sibling rivalry.

Of course, I've come across countless people who struggle with their siblings and a fair number who haven't spoken to a sibling in years but, despite all this, I've never really come to grips with what a powerful and challenging relationship this is. Now, having read psychologist Dorothy Rowe's book My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bonds – I'm a bit worried about David Miliband.

According to Rowe, if David should lose today's Labour leadership election to his younger brother Ed, he could suffer not just a political disappointment but also a profound feeling of annihilation. Rowe's book describes brilliantly the sense of complete invalidation that's brought about by discovering "your life is not what you think it is". In sibling terms, this is when the seemingly inviolable world order that is established in childhood is proved to be a construct, a mere interpretation and – horror of horrors – little brother has a different way of seeing things.

It's well known that in the family context you are both your self and your role. If you're the eldest, as I am, you might get used to being more powerful, for example, or wiser, or the one that knows best how to work the TV. Then you grow up and discover your younger siblings know things all by themselves. Even more annoyingly, they go off and get a life of their own and it's all very unsettling. Plus, each sibling interprets the situation so differently. The older might think the younger owes them for spoiling their idyll, that brief lovely moment of being the only child, but then it turns out the younger thinks the older owes them – for always being the first to do everything.

So you adapt and pretend to be grown-ups. The trouble is, when it comes to siblings, you're not actually grown up – you get thrown back into your childhood position. As Rowe points out, just as you can become like a child again around your parents, you can become a child again around your siblings. Some of this is lovely – you can laugh at childhood jokes, for example, in a way you just can't laugh with anyone else. But some of it is hard too. Rowe points out that there are difficulties in sibling relationships that can never be resolved – and this is a great insight in a world where we basically believe in the self-help quick fix. When you're with a sibling you can never not be older or younger and everything that means in terms of the family dynamic. Then there's the fact that if you were labelled as the "good" one or the "wild" one, that will always be with you as well.

One of the problems with sibling relationships is that they often look and feel like friendships – but Rowe calls siblings "dangerous friends". The truth is siblings aren't really friends at all. Like it or not, a friend is disposable and a sibling isn't. Yes, you can resolve never to speak to a sibling again, but that will leave a painful gap that will take just as much living with as the sibling ever did. This isn't true of normal friends, whom by and large you can move on from. Before you have a row with a sibling you need to consider not just the immediate consequences but the next 40-odd years. Siblings, like dogs, are for life.

David and Ed have pretty much pooh-poohed the whole sibling rivalry thing in the course of the leadership campaign, David saying definitively that "politics will never come in the way of family". The thing is, will family come in the way of politics? Having read Rowe's book, I find myself thinking it will be better for Labour if David wins. A younger brother could work for an elder. I'm not sure if it would ever stick the other way around. They're called sibling "bonds" for a reason.