Family dynamics and sibling roles are forged in the fires of childhood; they are defined and refined as we graduate from sharing toys to text books to the names of good dry cleaners. The roles are both tacit and concrete, shifting just a degree or two as you get to know each other all over again in adulthood.
And whether you're the responsible one, the practical one, the joker, the peacemaker or the black sheep, that's the part that you will play a variation of for life. When you're all in the same room, you can slip back into that role like it was only yesterday that your brother hid your dolls' house in the back room of the shed because his foot was battered and bruised form stepping on tiny, spiky bits of doll-sized furniture.
I am the youngest of four and I've accepted my lot. And it's really rather a fantastic lot. Not that I'd ever actually tell them this, you understand. Because even though they've been my greatest tormentors at times, they are also some of my favourite people on this earth.
They're the anchor to my childhood and have had a huge influence on the shaping of my character (and certain scars, both physical and mental).
Sibling relationships are layered with bonds that have withstood time, tantrums and toy fights that escalated rather quickly.
And though I am ostensibly an adult, something strange happens when the four of us are in the same room at Christmas. I suddenly sprout pigtails from the sides of my head and my wallet becomes full of Monopoly money that is no good in this town.
I spring into action when I hear the words 'run out to the washing machine there and put it on spin, I'll time you' or 'get the milk out of the fridge, you're the youngest'. And let's not forget the classic of 'stop hitting yourself, stop hitting yourself'.
I'm in my thirties and I'm still hitting myself, hitting myself.
On any other day a more normal adult-sized service is resumed but Christmas Day is reserved for sibling jostling, playing Brussels sprout football across the table and jumping out from behind the door to frighten any, and preferably all, members of my family.
It's part of my Christmas tableau. It's the one day that we are together, reminiscing and creating new memories that one day I'll take out, brush off and turn around in my mind's eye like a glittering bauble.
And these relationships can be just as fragile as that very bauble. So if your sister uses the last of your special vanilla coffee pods on Christmas morning or your brother turns off the light while you're in the bathroom and laughs maniacally outside the door while you hurl abuse at him, remember that some day you'll look back and know that these imperfect days were the perfect days.
Because siblings aren't just for Christmas, they're for life.
And this Christmas I know that I'm going to break my thirty second record of tearing down to the washing machine and back. How do I know this? Because I've stolen my sister's new runners. Don't tell mom.
Are you spending Christmas with your family? Does the thought of 24 hours of Trivial Pursuit with added sibling rivalry fill you with joy or with terror? We'd love to hear your thoughts, survival tips and about that time that you ripped the head off your sister's Sindy because she hit you in the face with a snowball.