1. First of all, they're automatically your oldest friends since they've been around for all (or nearly all) of your entire life.
2. Which means you've developed a system of inside jokes that is decades in the making.
3. And you can kind of be intimidating and/or alienating for others to be around.
4. They are the only people who can fully understand why that otherwise innocuous thing that mom or dad did was actually THE MOST ANNOYING THING EVER.
5. And they're the only ones with whom you can lovingly make fun of the family, because, you know, they're actually part of it.
6. Essentially, you are all each other's full-time family therapists, always on call.
7. They have the rare ability to fill in the gaps of your childhood memories.
8. But, inevitably, there are some details that you are still arguing about and grudges you are still holding onto.
9. You never know how to contribute in a conversation of people complaining about their families.
10. And you can't relate to the idea of dreading going home for the holidays.
11. When you DO go home for the holidays, you tend to immediately fall onto the couch and sink into sibling isolation.
12. All of their friends become your friends, and vice versa.
13. And a lot of their friends become your crushes.
14. You get to rely on ingrained birth order roles, which can be a blessing...
(Like when you're unemployed and your big sis spots you some cash.)
15. ...but also a curse.
16. You give each other the best gifts, because you've had a lifetime of training.
17. And you're untouchable in games like Catchphrase because you've developed an impenetrable shorthand.
18. They keep you grounded in the way that only someone who knows all of your embarrassing secrets can.
19. Which means they know the quickest ways to rile you up.
20. And you guys basically have zero personal boundaries with each other.
You used to take baths together, after all.