IT WAS my little sister's birthday this week. I say little, but she was 24 on Tuesday so I should probably drop the "little" and start saying "younger".

Running the risk of sounding like I'm trying to be Carrie on Sex and the City, with her rhetorical questions - it got me thinking that sisters are funny things, aren't they?

You can call them every name under the sun, steal their make-up, rush out of the house wearing their best jumper and borrow money off them that you never intend to pay back.

But if anybody outside the family even dares suggest they have a fault, you'll want to beat the living daylights out of them.

I'm the middle sister in our family, with four years between each of us.

But on the battlefield the age difference didn't matter - we were even.

As children, my mum regularly had to wade in to stop full-scale no-holds-barred sister fist-fights.

A particularly memorable one ended in a plant pot up-turned on someone's head.

But as you get older and move away from home you start to realise how much your sisters mean to you, and how much your personality is defined by being a sister.

True, sisters annoy you, they interfere in things that don't concern them, they tell on you, criticise you, take out moods on you, they borrow your stuff, break your stuff, and monopolise the bathroom for hours on end.

They wield knowledge (and have photographic evidence) that could crush your reputation and leave you a laughing stock to your mates and colleagues in milliseconds.

But if anything in your life goes wrong, your sisters are there, defending and protecting you.

They know your best and worst sides and they love you anyway.

A sister will tell you if you look a bit podgy in that new dress when friends will "white lie" that you look great.

You can be boring with sisters when with even the best of friends you have to make an effort.

And there's not much in this world as enjoyable as mooching around a department store make-up counter testing lipsticks on the back of your hands in companionable silence with a sister.

As Canadian author Charlotte Gray said: "We may look old and wise to the outside world. But to each other, we are still in junior school."

How true that is.

And before you complain that I've turned into a softie- remember, it is Christmas.