1. You must always take your sister's side in an argument.
2. Sometimes you won't understand why you're on that side. But that's OK. You don't need a reason for everything.
3. If your sister tells you a secret, you mustn't tell anyone.
4. Even your mum.
5. Especially your mum.
6. You must never tattle tale on your sister.
7. Even if she did something really bad.
8. Sometimes you will sometimes bicker amongst yourselves. That's fine too.
9. But if you were in the wrong, you must be brave enough to apologise.
10. Even if you're not in the wrong, you should probably apologise. Sisters don't hold grudges.
11. You must tease each other often. Sisters need to be able to laugh at themselves.
12. In the holidays, you must spend at least one day snuggling.
13. And one day taking ugly selfies.
14. Every now and then you must co-ordinate your outfits.
15. Especially if you're wearing denim.
16. You must never reveal how much food you're capable of consuming together to any non-sister.
17. When you're children, the older sister must fast forward all the rude bits of TV shows.
18. When you're grown ups, the younger sister must make fun of the fact that the older sister used to do that.
19. If someone hurts your sister, you must hate them.
20. If someone dumps your sister, you must hurt them.
21. You must like every photo your sister posts online.
22. And if she looks reeeeeeally beautiful in them, you must tell her.
23. As a sister, you are always on call. If your sister needs you to talk to her while she walks home in the middle of the night, you must wake up and do it.
24. You must never ever utter the phrase, "Told you so."
25. You must support even the silliest of your sister's life plans.
26. And if your parents doubt said plans, you must put in the hours to convince them that your sister knows what's best for her.
27. If you live far apart, you must make time to Skype each other.
28. But you must never take screenshots when your sister's face freezes in an ugly position.
29. Being a sister means being someone's first priority. If your sister needs you to be somewhere, you must cancel all plans and go.
30. But you must never cancel on your sister.
31. If anyone bitches about your sister, you must defend her. Sisters always have each other's backs.
32. When you bake together, you must double the quantities so you can eat one raw cake and one normal cake.
33. You must always cover for each other. No questions asked.
34. You must share everything you own. Even when you no longer live together, your stuff is her stuff and her stuff is your stuff.
35. That said, you must always ask permission before borrowing anything belonging to your sister.
36. And you mustn't lose anything.
37. You must analyse every family event in intense detail together.
38. You must never tell non-sisters you do this.
39. If you're the older one, you must proof-read your sister's homework, teach her how to avoid following your parents rules, and punch any boy who hurts her.
40. If you're the younger one, you must cover for your sister when she stays out late, listen when she moans about work, and punch any boy who hurts her.
41. When you go out to eat, you must split the bill. Unless one sister is going through tough times or is a student, in which case the richer sister must pay.
42. You never need to ask permission to climb into each other's beds for a cuddle.
43. You must not go through any big life developments without telling your sister. In fact, where possible, you should keep your sister up-to-date on your every move. There's no such thing as too many WhatsApps.
44. If your sister does something bad, you must listen to her side of the story before passing judgement.
45. And even if it turns out she was in the wrong, you must love her anyway.
46. You must say "I love you" to each other regularly. Preferably once a day.
47. If your sister tells you a story about a new friend, you must Facebook stalk them until you feel like you know them.
48. You must give your sister your Facebook password to make this process easier.
49. When your sister gets sick, you must send supplies.
50. You must never stop doing all the dumb childhood stuff that makes each other laugh.
51. You must vet each other's boyfriends.
52. And actually listen to what each other think. If your sister doesn't like him, he's gotta go.
53. You must be there for each other through every break up.
54. With chocolate.
55. You must always be honest. Sisters don't lie.
56. That said, you must be kind. If your sister's new haircut looks gross, there's a way of telling her.
57. You must always trust each other. If you get silly drunk, you must trust that your sisters knows when it's best to send you home.
58. You must sing and dance together badly and often.
59. You must send each other post. Letters from sisters are the actual best.
60. If your sister gets sad, you are obligated to listen. Even if she cries so much she gets kinda snotty.
61. If you're the little sister, you must copy your big sister.
62. If you're the big sister, you must accept that little sisters copy big sisters and see it as a compliment.
63. And you must let your teddies catch up regularly.