1. When you're dating someone new and you're about to embark upon a huuuuge social media stalk.
2. When you're trying a new shade of lipstick and you need an honest opinion.
3. When you're about to tell your mum something ~big~ and you need someone to lay the groundwork.
4. When you mess up and need someone to cover for you.
5. When you're walking back late at night and you need someone to be on the phone.
6. When you have nothing to wear.
7. When you need someone to support your drunken life plans.
8. When you're so hungover that all you can do is grunt.
9. When you curl up on the sofa to watch O.C. episodes back-to-back.
10. When you're forced to attend a family event that you really don't care about.
11. When you need to analyse said event on the way home.
12. When you think you know what you think but you're not sure you know what you think.
13. When you need to rehearse a scary conversation with your boss.
14. When you've taken a selfie so ugly that no one can see it but someone must see it.
15. When you wanna coordinate outfits.
16. When your family are being 100% annoying and you just need a little support.
17. Followed by a little bitching sesh.
18. When you know something you're not supposed to tell anyone but you have to tell someone.
19. When you've just finished an amazing book and you have to tell someone about it.
20. When you've just finished an amazing film and you have to tell someone about it.
21. When The Parent Trap is on TV.
22. Actually when any sister film is on TV and you need to work out which sisters you guys are.
23. When you find out some salacious gossip about one of your old teachers.
24. Or one of your old neighbours.
25. Or your best friend from year 5's mum's new boyfriend.
26. When you've just run into someone from 10 years ago and they've changed their hair.
27. When you see a dog with a funny face.
28. When you need to talk about how weird your parents are.
29. When you need to tell someone the ugly details of your relationship.
30. When it's Christmas.
31. Actually, any time you go back to your hometown.
32. Especially when your mum sends you out to do a boring chore, because you know your sister will turn it into an adventure.
33. When you're so drunk you need someone to remind you how to put your key in the door.
34. When you're so mad that you need to shout at someone without being scared of accidentally getting into a fight with them.
35. When it's your birthday and you need to work out your goals for the next year of your life.
36. When it's her birthday and you need to remind her of all the embarrassing things she's done.
37. When your teddy misses her teddy.
38. When you're about to eat your own body weight in take out.
39. When someone is being mean to you, and you need your sister to point out all the ways you're better than them.
40. When someone is being mean to your sister, and you need to be meaner back.
41. When you massively judge something totally random, but you can't tell anyone because they'd think you're too mean.
42. When you're ill.
43. When you're so upset you just cry snot.
44. When you want to cause mischief.
45. When you need someone to lie for you.
46. And every time you need a proper hug.