For Libby, Sarah’s sister. Happy 21st birthday. You have a good sister.

There’s a family-famous picture of you from when you were born. I sit in a hospital chair, my unruly curls running wild, and cradle you, a bald baby, in my arms. I look down at you with awe in my eyes. It’s a sweet photo.

Later, during a fight as teenagers, I tell you I should have thrown you out the hospital window when I had the chance.

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You fight. At first, you are children and the fights are for no reason. Someone took a dolly, someone pinched the other, someone ate from someone else’s plate. One time I strung your Barbies up on the ceiling fan and told you they’d all killed themselves. Another time, I convinced you our two best friends, brothers, were possessed by demons. You hit me on the back with a baton so hard I had a red welt for weeks. When someone else at school bullies you, I show up at your lunch table and stare them down until they cry. You once punch someone in the face for me at summer camp. We can say whatever we want to each other, but everyone else better step off.

We start to fight about different things. Your behavior, your grades, your friends. I am older so I become the third parent — or in some regards, the only parent. And yet you still confide in me and I in you. You come visit me in college and I get you high for the first time under a statue in the Public Garden. After a particularly good date with a hot guy, you text me a winky-face emoticon when I ask how it went. When I get back to my old bedroom, you and a male friend have plastered pictures of yourselves drinking pina coladas by the pool all over. I’ve been in school in freezing cold Boston. So you know, it’s hilarious.

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To be a sister is to be part of a pie graph, to a be cog in the Trivial Pursuit wheel, to be unfinished. Many times, I’ll show people a photo of my sister, or they will meet her and they’ll tell me, “Oh my god, you make so much more sense now. Your whole personality makes so much sense.”

We exist in comparison to each other. Isn’t it strange to have someone else out in the world who for 18 years mostly lived a similar life to you? All your relatives are the same relations to them. All your parents have ever done to you, was probably done to them as well. Or at least, they know about it. They were made with the same ingredients and they can turn out so, so differently. I have friends with sisters who are their replica — like someone took the same parts into a workshop and molded two of a kind.

My sister is more like someone took the same parts and then put them in a washing machine, mixed them up and then lined the results up in a different order. Once, a man who’d known us both separately for years only realized she was my sister when she made a particular facial expression. “Gaby Dunn makes that face,” he’d said. “Do you know her?” My sister had laughed and laughed.

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Being a sister means feeling responsible for someone else all the time. It means having a built-in best friend and someone who can, during an argument, call you out for shaving your eyebrows off in the fifth grade. It means communicating in inside jokes and shorthand and pointed looks when mom starts in on the red wine. It means fighting about whether or not “Glee” is a good show, but being able to quote every episode of “Arrested Development.” It means defending someone, even as you slap them upside the head for being an idiot. It means waking up at 7 in the morning to a one phone call from jail. It means wading through photos of outfits so you can help them pick the best one for their job interview even though you are at work. It means using two forks on one big strawberry cheesecake at 12:30 p.m. while you talk about why dating sucks. It means crying on the phone. It means sharing hairbrushes and a bathroom and shampoo. It means coming out to them and having them say, “Yeah, so? I love you. Who cares?” It means tolerating family vacations and straightening the back of the other’s hair so it’s perfect and wandering around the mall for no reason because it’s another Saturday in high school and everything sucks anyway. It means growing up with someone and watching someone else grow up.

And mostly, ideally, it means you are never alone.