One of the reasons you have more than one child (apart from 'we've got all these baby clothes now, so we might as well use them') is that you think it might be a bit lonely being the only one. So you introduce a new brother or sister, and then watch, horrified, as war breaks out. (Hell hath no fury like a toddler usurped by a new baby.)
But as the years go by, battle lines are re-drawn. Sometimes siblings fall out big time. But most of the time, as they grow up, they learn to get on. They join forces. They have, after all, got a lot in common – genes, a surname, and flawed, irrational parents.
(I remember a friend of mine telling me about the time her two sons,six and eight, were too busy fighting to go to bed. She burst into their room shouting she'd had enough, she didn't want to hear any more arguing, and if she had to come upstairs one more time there would be serious trouble. As she left, she heard a scuffle, and then a small voice urgently hissing, "Stop it. You'll only make her worse.")
So most of the time, in the general muddle of family life, siblings get on.
But sometimes, when they reach their teenage years, the united front seems to crumble. They start needling each other. They point out failures and shortcomings. They may even start fighting physically.
Psychologist Dirk Flower believes this is perfectly normal behaviour at this age. "It's part of being a teenager. They're trying to define themselves, and find the other person irritating. Separate them, but let them be. It will generally settle down in a couple of years."
My teenage daughter says, "I don't think parents should worry. Sometimes it gets a bit much being stuck in the same house with each other day after day. You need a bit of space. And siblings aren't like friends. You say exactly what you think to each other. And sometimes you're not in the mood for that. You just want to be left alone."
You probably need to check that you're not doing anything that might fuel resentment between them – which could, given the emotional way teenagers view the world, be absolutely anything (always buying his favourite biscuits, letting her stay out till midnight). But there may be nothing you can put your finger on.