1. She expects you to share. In big families, family-style is just the norm. So always, always order a second fork with your dessert. (Just don't eat the last bite of mine.)
2. She'll happily clean out your closets. After years of complaining about hand-me-downs, I've learned to love them and can't get enough. So when I say I like your super-soft college tee, don't be surprised when I actually wear it. (I have no shame.)
3. She wants at least three kids. (Not two. Not one.) No matter how much I hate on my sisters at times, I can't imagine my kids growing up without the same support system. This is a nonnegotiable, so move on if you're not into big families.
4. There are two things she'll never admit to. (1) I do not have middle child syndrome, and (2) I love both of my sisters equally (on and off the record). So don't bother asking.
5. She'll totally understand your other priorities. I grew up sharing the time and adoration of my parents, who (bless them) tried to dole it all out equally. If you need to skip a holiday at my folks' house to take a work trip or grab dinner with your guys even though we'd planned to stay in, I get it. As long as I know you'll be there for me when I really, truly need you, I'll take whatever time I can get.
6. She'll baby you. My older sister is fiercly protective of me, and I'm the same way about my younger sister. My weird maternal instinct manifests itself in random ways, so for god's sake, WEAR YOUR HELMET!
7. She will compare you to her siblings' partners. I can't deny that I've sort of always wanted to be my older sister. She's the shit, and her husband is too, which means I'll constantly compare you to him. (And so will my parents. So good luck!)
8. She demands equality and fairness. Constantly feeling shortchanged as a child makes me hypersensitive to wrongdoing—my moral compass is especially well tuned. This will work out to your benefit when I insist on splitting bills fifty-fifty, but come back to bite you if you step out of line. I will call you out immediately.
9. She's an excellent teacher. I taught my little sister how to escape from her crib and how to tie her shoes. I'm a natural—listen up, and you, too, shall learn.
10. She leads by example, so take the hint. My little sister still copies (almost) everything I do. I'm used to it—so when I put my dishes away immediately after using them, I expect you to follow suit.
11. She's totally cool spending time by herself. Growing up, I happily played by myself whenever my parents were caught up with other children. In fact, I actually relished in the peace and quiet. So you do your thing, I'll do mine, TTYL.
12. She is the devil's advocate, incarnate—so she won't just take your side. Having always played two roles (big sister and little sister), I'm used to assessing situations from every angle. Which means I'm programmed to give you sympathy before my unwavering support.
13. She's supremely flexible. When I say, "I don't care if we go out or stay in," I mean it. Whatever you want! I'm used to going with the flow (and having someone else take the lead—my older sister always did, and still does).
14. You'll need all her siblings' approval to secure her approval. Double or nothing!
15. She's super sensitive. Serious talks choke me up, mostly because crying was the only way to guarantee getting attention as a kid. I'll feel silly when this happens, but I can't control it. And I'll get even more upset if you don't take me seriously.
16. She's super hard on herself. Having worshipped my older sister, and always compared myself to her, I hold myself to high expectations. So I need you to reassure me (about my outfit, wit, charm etc.), and to do it often.
17. She'll rarely ask you for help. Because the oldest child always earns attention while the younger one always seems to need it, middle children have to fend for themselves. So I'll spend two hours hanging a picture with a shoe heel and a nail before I even think to borrow your hammer. That said, feel free to offer it.
18. Don't you dare insult her siblings. When my sisters team up on me or leave me out, I'll expect you to take my side. But that doesn't give you permission to lead the roast. They're my sisters, so I'll have their backs—always.
19. She's an incredible partner. I've practiced nurturing my little sister and being nurtured by my older sister. I know how to be the perfect partner in crime.
This article originally appeared on Cosmopolitan.com. Minor edits have been made by the Cosmo.ph editors.