1. The first time you decide to hate someone because they were mean to your sister.
2. The first time you bitch about your family together.
3. The first time you have a proper shouty fight.
4. The first time you make up after a proper shouty fight.
5. The first time you copy each other.
6. The first time you acknowledge copying each other.
7. The first time you throw each other a "let's get out of here right now" look.
8. The first time you act indefensibly weird in public together.
9. The first time you know what your sister is going to say before she says it.
10. The first time you lie for each other.
11. The first time you sleep in the same single bed as adults.
12. The first time you coordinate outfits as adults.
13. The first time you try a weird new beauty treatment together and your faces go all red and blotchy.
14. The first time you eat a meal, don't feel full, and then eat another meal together.
15. The first time you live in different cities.
16. The first time you miss each other so much you cry.
17. The first time you write each other letters.
18. The first time you deal with a family tragedy together.
19. The first time you drunk text each other.
20. The first time you get drunk together.
21. The first time you get through a killer hangover together.
22. The first time you bitch about your parents together.
23. The first time you try to understand family politics together.
24. The first time you stand up for each other.
25. The first time you take a goofy selfie together.
26. The first time you invent a secret handshake.
27. The first time you invent a secret language.
28. The first time someone else tells you that you look like your sister, and you swell with pride because you literally can't imagine a more beautiful woman.
29. The first time you reminisce over something from your childhood and laugh and cry at the same time.
30. The first time you look after each other when you're ill.
31. The first time you know your sister is upset before she tells you.
32. The first time you share a friend.
33. The first time the younger one gives the older one advice and it feels weird, but nice.
34. The first time you realise that you are equally smart.
35. And equally pretty.
36. The first time you irrationally judge the same totally normal behaviour and can't really explain why.
37. The first time you cry at the same point in a film together.
38. The first time you meet each other's S.O.s.
39. The first time you sit in silence and listen to your sister complain.
40. The first time you argue about who your parents love more.
41. The first time you sing your hearts out together.
42. The first time you geek out over the exact same thing.
43. The first time you talk about sex together.
44. The first time you watch an entire box set in one sitting together.
45. The first time you watch a film together and fast-forward all the bits you hate.
46. The first time you spend your birthdays apart.
47. The first time you spend Christmas apart.
48. The first time you vow never to spend the holidays apart again.
49. The first time you watch a movie in synch on a plane.
50. The first time you watch a movie in synch over Skype.
51. The first time you hack into each other's Facebook accounts so you can stalk each other's friends.
52. The first time you snuggle all day.
53. The first time you blank someone because they were mean about your sister 10 years ago.
54. The first time you need someone to talk to, and you realise your sister is the only person in the world who fits the bill.