Nine things only people with a sister will know to be true.

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Sisters have the potential to be annoying. They read your diary (back in the day). They steal your clothing (to this day). But in the profound words of noughties girl band S2S:

“Closer than my closest friend, someone who will be there till the end. My sister, sister.”

Yeah, they were onto something – and anyone with a sister (I’m lucky enough to have two of the best) will know they really are the greatest kind of friend you’ll ever have.

 1. Nonsense nicknames.

Over the years of growing up together, you’ve accumulated an extensive record of nicknames for eachother. “Mud slaps”, “Mootles”, “Kitty Siesta”. Like codes passed between Russian spies, they’re likely to make no sense to anyone but you.

2. Hot and cold.

It’s a truth universally acknowledged that a sister is the only person in the world you can hate passionately – despise, even – one minute, and fall about laughing hysterically with the next.

3. Bribery is rife in the sisterhood.

Anyone with a sister learnt the benefits and pitfalls of emotional bribery long before their peers.  Favours? They come at a severe cost.

“Can you please drive me to the station?”

“Sure, only if I can have your Backstreet Boys CD for a week.”

“You borrowed my perfume last week so I’m taking your foundation.”

“But I let you wear my favourite jeans and you stained them.”

It’s a lifelong struggle between who owes who more, and both of you will constantly feel hard done by.

TOMORROW IS NATIONAL @BELLAHADID DAY YAAAAY FOR DA BOYZZ 18 18 18 ???? ????

A photo posted by Gigi Hadid (@gigihadid) on Oct 8, 2014 at 8:53am PDT

4. There are rules when referring to your sister.

You’re allowed so say what you wish about your dear sister, whether it’s good or really quite vile. But anyone who even THINKS about so much as agreeing with you when you’re trash talking her can back the hell down. Nobody talks about your sister like that… Except for you.

5. You can be 100 per cent honest with your sister.

Honesty among friends is great, but there’s a limit to just how much truth you can slip in there. Only a sister can tell you those pants somehow make you look like the genie from Aladdin. Or that your feet smell putrid whenever you remove them from a particular pair of boots. Thanks sis.

Click below for photos of sisters Jessica and Ashley Hart. Post continues after the gallery

6. They’re excellent allies.

Whether you’re banding together nobley to fight your parents about being allowed to stay up late to watch Big Brother Uncut, or you need assistance when someone spills a drink down your legs when you’re at the pub, your sister is the first to back you up and she’ll do it regardless of whether you were right or wrong.

Image: iStock

7. Fists have been raised.

You've never been in a fight in your entire life, that is, except for when it comes to the vicious foot fights with your sis over who was taking up the most room on the couch. Or the scratching and hair pulling that occurred when she "stole" the last of the Nutella from you. Or when you kicked each other when she hogged the doona while sharing a bed at your grandparents’ place. Yeah, apart from that, you've never been in a fight.

8. They know too much.

Every boy you had a crush on. Every time you humiliated yourself in the school quadrangle. All of the occasions you cried when you lost at Monopoly. The fact that your bike needed trainer wheels for twice as long as the average kid. Your sister is an encyclopedia of knowledge - the A-Z expert - when it comes to every embarrassing thing you've ever done. It's ok though, you could write the book called My Sister’s Embarrassments.

Image: Di Novi Pictures/Warner Bros. Pictures

9. Best friends 4 life.

Anyone with a sister knows that you were born with the greatest friend you ever could’ve hoped for. Someone who’ll coach you, console you, laugh at you, laugh with you, care for you and love you unconditionally.

Happy International Sister’s Day!