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Your sister finds a theme song for every year from the day you turn thirteen.
That year, it's a Justin Bieber song and you hate your life unequivocally. Every time she finds herself a boy to 'love' she runs around rapping, badly, "I had my first love / when I was thirteen," as if she has any idea what love really is or what it's going to do to her when it hits her.
You don't, either, of course.
But she sings it anyway and you moan as she hits the chorus and screeches "baby, baby, baby," over again, but really, you can't complain, because Mabel is happy and convinced that this is her year to shine. What she doesn't seem to realize is that every year is her year to shine.
She is the outgoing, the beautiful, the sweet; she loses the braces and gains (to your supreme distaste) a training bra. You, on the other hand, have a voice that cracks more easily than an egg on a brick wall and a love for books that most people consider worthy of genuine concern. You have always been a little jealous of Mabel, but you never say so. She is the sun and you are the moon, and she burns brighter than at every moment save the ones of the lunar eclipse.
When you're fifteen, it is the abominable Taylor Swift song. You spend the summer in Gravity Falls again, and she wriggles her toes in the sand at the lake and exchanges her crazy sweaters for crazier tank tops, and you spend all your time ogling Wendy fresh home from college with her new pixie cut, who kisses you behind the shack because it strikes her fancy.
Mabel kisses a local boy with her toes in that sand and thinks of a merman she found in the pool when she was twelve.
That is the year you find out that Waddles is actually Mrs. Waddles and now Momma Waddles, and Mabel sobs uncontrollably as you find homes for all of the piglets as far away from Old Man McGucket as possible. It's also the year when Gideon starts flipping up Mabel's skirts, and she starts learning karate, and you find a village of kelpies at the edge of the lake and almost die.
That summer you both leave feeling fulfilled, between kisses and kelpies and the ever-present vampires, ghouls, bridge trolls, and aliens, and Mabel doesn't pay attention to the half of the Taylor Swift song that speaks of heartbreak. That only comes six months later during a winter break visit when she protects the diner from a werewolf and finds her local boy kissing... another local boy.
You wonder if she ever really loved him and hope not. But Mabel loves people like you love books: immedately, unconditionally, and infinitely.
These are the days that you don't feel quite so jealous of the sun that shines so bright she burns herself.
When you're sixteen, she sings the Sixteen Going on Seventeen song from the ancient Sound of Music in her sweetest voice, but she mixes up Rolf and Liesl's words to make something cynical and sad. Gideon bothers her less and less until he doesn't, anymore, but you don't think it's because she has her green belt now.
For the first time you start to mind her singing, not because it annoys you, but because her own words tear her apart.
And that summer at Gravity Falls, Mabel goes to the movies with her local boy and his local boyfriend and comes back with a suspicious red bruise on her neck.
Wendy brings home a new boyfriend and doesn't look at you like kissing you is funny; she doesn't look at you at all. When they sneak off into the woods, you walk the other way, and end up with your toes in the sand, sitting beside your sister as the sun sets.
For the first time in years, she drapes her insanely-long brown hair across your shoulders, the way she used to do when you got upset. And she stares at the dock where a fuzzy-lipped merman (boy, really) gave her her first kiss, and you watch her wonder where all the magic in romance went. She uses her own long hair to dry her tears. You put your arm around her shoulders and watch a familiar shadow beneath the softly lapping waves.
By the end of the summer, Mabel's local boy is missing and the kelpies give you sharp, knowing smiles from beneath the waves.
That's the summer that you give up on being the Mystery Twins and put the journal away.
When you turn seventeen, she insists not on one theme song of the year but on a hundred, because there are a thousand songs about being seventeen and she is determined to live all of them. Suspiciously, she never sings the lyrics about love.
And that summer at Gravity Falls, Mrs. Waddles goes missing and Old Man McGucket looks disturbingly well-fed. While Mabel is out looking in vain for her pet pig, you find Stan cold and unmoving in his room, a wad of old bills clutched in his fingers.
It's worse than any monster that you've ever encountered here, and the two of you get bussed home straight after the funeral, leaving Wendy empty-eyed with a cigarette and Soos with tears pouring from his eyes.
Mabel cries on your shoulder about these things and so much more--about boys and beaches and big sweaters and the way people look at her--and you try to be a good brother, and fail.
There are so many songs about being seventeen, and this is the exact opposite of what either of you ever expected.
A shooting star passes the next month, crosses the constellation you're named for, and you hear her wish to be able to let it all go.
On your eighteenth birthday she pulls an old sweater out from the depths of her closet, and years after she's stopped wearing them, they finally, finally fit. Her hair is short now and she doesn't wear skirts and her braces went long, long ago, and yet she still looks more like the sun you remember her being than she has in a very long time.
She has something behind her back. Before you can fully look up from your book, she has jammed an old, dirty hat on your head, and you pull it off briefly to see the blue tree stenciled on the front and you can't help but smile.
The two of you drive all the way to Gravity Falls and stop at every gas station and little local diner along the way, and when you finally get there you hug Soos and steal his keys and run around the dusty, terrible shack like you're twelve again. You chase her like you used to years ago, play attic golf, walk through the woods in the dark and scare a monster or two. You find the book and page through every entry. You pat the furniture like it's an old friend, and Mabel kisses the ceiling beams. When you leave the shack with a book and a sack of old memories, Mabel drops a bottled message into the ocean and you leave a box of old love letters titled 'farewell' at Wendy's doorstep.
And then the two of you get in the car and google mystery towns, and drive.