JUST when you think you know your children, along comes a little something to make you reconsider.

Little things don't come much littler: a virus. The common cold led to an outbreak of uncommon behaviour in our household.

There was a rare display of sibling affection between that usually incendiary combination: older sister (12) and younger brother (nine).

On Friday evening, there was my daughter: laid horizontal on the sofa, hacking like a 60-year-old, 40-a-day coal miner. My son appeared from the kitchen and closed on the patient. I presumed he was moving in for the kill, or at least to gloat.

Instead he placed a brotherly arm around her and said, "Ahhhhh."

I flinched, fearing the reaction from his sister. With her being a she, you have to expect the unexpected. The two annual kisses she receives from him (on her birthday and on Christmas Day) are greeted by "Urgh! Boy germs!" and a shrinking back as if from a Dalek's embrace, or by a scene of such sisterly tenderness that even the writers of The Waltons would resist it as outrageous. Any other physical contact by him is regularly returned by her, with escalation (a thump for a pinch, a kick for a whack, a beheading for a mutilation). Or she can confound us with a soft "Don't let's fall out" and reach for the moral high ground.

This time – phew – she selected the non-nuclear option. "Wuhhhh," she sniffled. "Thank you."

By Saturday night, the little fella had succumbed to the big girl's bug. As I was tucking her in, his coughs were rattling the adjoining wall. "Ahh," she said, adding, "Poor chap," without sarcasm.

The rest of the weekend was freakish, as if they'd swapped personalities. He'd be mean one second, yarling that she'd taken his Nintendo game; the next he'd offer some of his chocolate. Her usual love-him/ hate-him mood swings vanished, replaced by sweet consistency.

My wife and I were in a quandary even without the flipped behaviour. Illness didn't exist in my house when I was growing up. You went to school aged five and stopped aged 18, whether ill or well, alive or dead. My wife endured the same in her youth. She was top of the class in the School of Tough Love and went on to graduate with first-class honours from the University of You'll Either Be Dead Or Better By the Morning.

In her family, kind words are offered only in cases of the amputation of a leg or the onset of the latter stages of terminal illness. Even then, the best you can expect is, "Think of the money you'll save on shoes" or "Lucky you! No Christmas shopping next year."

The standard remedies dispensed by her father are, in the event of almost certain imminent hospitalisation, (a) an aspirin, and if the undertaker appears at the front door with a tape measure in hand, the addition of (b) a cupful of dissolved Oxo to wash the tablet down.

My wife has mellowed with age and the arrival of children. She sometimes drives an ailing offspring to school rather than making the child trek through the rain. Sweet. Her ministering to the sick includes paracetamol and lemon: but she mainly practises bed-based cure, although the precise nature varies with the time of day.

"The best place to be if you're ill is in bed, resting," she announces, early evening. "The best thing to do if you're ill is to be out of bed, doing things," she says in the morning, particularly on a school day. I wouldn't argue with that, although I might add a little sugar to the boiled-up citrus fruit and pass the liquor off as homemade lemonade to make the paracetamol go down.

Come Sunday evening, I was getting concerned. I was about to trump the University of Exeter boffins whose research into animal behaviour this week found males are consistent and predictable and females do constantly change their minds.

A weekend with two sick children proved the cold virus can change personalities, possibly even for ever: but then Monday dawned and my son coughed with a mouthful of cereal, spraying cornflakes at his sister. "Euw! Put your hand over your mouth if you cough!" she shouted.

"You're not the boss of me!" he yelled back. They must be getting better. Normal service resumed, with no lasting damage.