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Horny Ghosts	By : PineWreths Category: +G through L > Gravity Falls Views: 10662 -:- Recommendations : 0 -:- Currently Reading : 0
Disclaimer: I do not own Gravity Falls, nor the characters from it. I do not make any money from the writing of this story.

Chapter List
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It only took the twins a half-hour of hiking and running through the trees to get to the edge of the lake Mabel had described. Sure enough, the cave was difficult to see, secluded and partially hidden by the hanging rootmass of an overturned spruce.
She ducked inside and her brother followed, Dip ducking his head to avoid the stony ceiling. Running water echoed from inside, but the interior was surprisingly well-lit; Water running through the soft rock of the cave had eroded little windows, most no bigger than a fist, in the mottled wall, and beyond Dipper could distantly make out the shape of the lake and a few tiny fishing boats.
His attention was caught by his sister giggling and disrobing; As usual, she hadn’t tossed on a bra, instead opting to take advantage of the volume of the sweater as concealment of the fact. Dipper grinned, appreciating the view as she laughed and splashed over to the small waterfall, the water almost making a musical tinkling as it hit the shallow pools below.
Dipper couldn’t help but stare at the beautiful crystals, and desperately wish he’d brought his own still-unfinished journal to do some sketches. It looked like they’d been formed of glass, their reflection so clear he could see his own birthmark clearly on his face, even with the water running over the surface of the gem.
He couldn’t help but run a finger along the surface of the gem, feeling an odd warmth as he drew his hand back; The fingertip was coated silver, which wispily faded into nothingness, as did the finger.
Dipper was so preoccupied that he hadn’t bothered to pay attention as his sister, her arms now invisible up to her elbows, carried over a cupped invisible-handful of water to splash on him.
Dipper jumped backwards, instinctively going to wipe his nose off and-
He missed.
Instead, his invisible hand, merrily screwing with his perception of where it had been, nearly poked him in the eye, causing Dip to swear and flinch as Mabel again splashed him.
Dipper groaned as he could feel his clothes getting soaked with his sister’s antics, but when he looked they were wet, but still clearly visible, unlike most of his body.
“Huh? Mabel, our clothes…”
“The effect doesn’t work on anything except animals, I think,” she said, ducking her head under the stream and causing it to vanish. Her invisible head spluttered and spat excess water, and he could hear her wringing out her hair as she said “Just fold and stick your clothes over by mine, behind that rock that looks like Grunkle Stan’s head.”
Dipper nodded, pulling off his shorts and shirt, and sticking them, his hat, and finally his boxers behind rock that looked uncannily like their Grunkle’s face. As he turned, a pair of pants and sandals went sailing past him, and Mabel, or at least most of her above the knees and below her bust, was standing there with patchily-visible arms on her hips.
Dipper smiled, and said with a murmur “Huh. Weird, but I also can’t say I dislike the view.” He could hear Mabel’s chuckle as she walked towards him a slinky swing in her hips and an odd tense note in her voice as she said “Well, does Little Dipper approve?”
Dip grinned, leaning his hips and his suddenly-straining erection forward as he said “You know he does.” He frowned, letting out a breath as Mabel said again with the oddly-tense voice “I know, do I?”
He looked down, letting out a little whimpering groan of frustration as he saw that one of his sister’s splashes had granted his penis the gift and curse of invisibility. He could feel the erection, but it currently looked like the region his shorts normally occupied had been crudely cropped out of existence instead.
Mabel’s oddly-tense tone finally dissolved into a “Snrk!” and then a guffaw of laughter at the look on her brother’s face as he whimpered “But-but-” with a slightly pathetic note in his voice. Then a mottled arm grabbed his, and he was yanked under the waterfall, the rest of him and his sister going transparent as she pulled him in for a kiss.
Dipper closed his eyes as she ran her tongue along his lips with a giggle; It wasn’t like looking around was going to do a whole lot anyways, but he could feel her warmth, her shivers as the cold water struck bare skin, her writhe against him.
“Let’s finish the job from earlier, brobro,” she said huskily as she guided him between her legs, and engulfed him for the second time that night.
This time, Dipper ran a hand down, running a finger along and around her clit as he plunged into her. Water splashed off her shoulders, crudely outlining her form as she moaned his name. Her legs wrapped around his waist, and Dipper removed his hands to cup his sister’s ass, leaning her gently back up against the smooth water-worn wall before thrust again.
He ducked his head down to kiss her breasts, but ducking his head around he kept seeming to miss them slightly before he could get a fix on them. Then Mabel shifting her weight keyed him in to her messing with him; That, and her giggling laugh, before Dipper finally caught her and engulfed a nipple, giving her a gentle bite of reprimand as the laugh morphed into a shuddering breath.
Finally, Dipper could begin to feel himself clench; He cursed inwardly a bit, as he was hoping the fun would be able to last a little bit longer, but the blueballs from earlier had other plans. Dipper reached down again, running a finger along her lips before massaging her clit with his thumb, and he was rewarded a few seconds later by the feeling of her clenching, her arms suddenly reaching around him to dig shallow tracks in his back as she sucked in breath through her teeth, making a strained little“yesyeyyesohgodyesyes-”
That sound drove Dipper to release, and as she stopped shuddering he came, feeling himself spurting into her and sending Mabel into a second round of shuddering breaths as she clenched and had a second, smaller orgasm. They hugged each other close, Dipper going soft inside her as they felt their breathing slow.
Mabel disengaged first, and Dipper felt her move, a soft wet noise, and-
Dipper had already thrashed away as he felt her sticky thumb across his forehead, swearing and going to wash off his anointed forehead as Mabel began cackling. After cleaning off his head, Dipper splashed back in the direction his sister’s giggling was coming from, saying “Hey Mabel, you know how you hate wet willies?”
Mabel’s cackle suddenly cut into a voice fraught with concern. “Uh, yeah?”
Dipper had just barely brushed the side of her head when she knocked his hand back with a squeal, laughing as she ran out of the cave. Dipper followed, laughing as well in the clear night air.

The ground turned out to be fairly soft loam, and pretty easy to walk on. He had worried about footwear and protection on their hike in, especially after he found that they wouldn’t go invisible like the rest of him, but now it turned out to be a non-issue.
Dipper felt weird being nude, despite being invisible. Mabel had always been the one more at-home with tossing off most or all of her clothes at the end of the day and dancing around home naked as the day she was born when they had the Shack to themselves. Dipper, meanwhile, had to consciously remind himself not to shower in swim trunks, and so he was lagging behind when he first heard the voices.
Mabel let out a muted giggle, and ran back towards Dipper, quietly calling his name. His outreached hand caught hers, and she gave it a reassuring squeeze. She must have noticed her twin’s slight hyperventilation, because she said with a reassuring voice “Relax, Dip. We’re invisible, remember? I can’t see a hair on your head, and neither can they.”
Dip started to nod, before remembering his condition and sheepishly said “Yeah, I guess.” Her hand tugged his, leading them past a group of hikers sitting by the trail, and onwards towards the town, specifically in the direction of the mall.

They cut through the parking lot, diving to one side roughly to avoid a car that, Dipper facepalmed in retrospect, couldn’t see us. Mabel pulled him close, giving him a reassuring loving kiss on the cheek, before pulling him inside the doors after a gaggle of teenagers.
The warm, toasty feeling from his sister’s body heat being pressed so close rapidly dwindled as a wave of fear hit him. There were hundreds, maybe thousands of people, and Dipper’s mind reminded him that, yet again, he was stark naked. Oh god, we’re a banana peel and a bucket of water away from never being able to show our faces in public ever again ohgodohodohgod-
Mabel’s voice interjected again, as she said quietly but firmly “Dipper!” He realized he’d been hyperventilating again, and ducking behind the mall map, took a few shuddering breaths. He could almost imagine her smile as he felt his sister’s hand on his shoulder. “It’s all right, brobro,” she said quietly, “Nobody can see us, nobody’s gonna see us, so we might as well have some fun, yeah?”
Dipper sighed, and in a somewhat-sulky voice said “Fine, if you say-” He cut off as Mabel’s unseen head said “Good!” before ducking down, and Dipper felt his knees go weak as she teased the head of his cock with her tongue. The healthy dousing of fear had shrunk Little Dipper to a pale shadow of his former self, but his twin’s ministrations soon had him fully erect. He reached out a hand for stability, running it through her silky hair as her head bobbed on him for the second time that night.
He could feel his vision narrowing when suddenly the lovely, encompassing warmth vanished, leaving the cold breeze of the mall’s AC to suddenly shock him into focus. He could feel his sister stand, and lean in to kiss him briefly on approximately his lips. It was technically the edge of his chin, but given that he’d almost poked his eye out earlier all by himself, Dipper wasn’t going to fault his twin’s aim.
Mabel had wrapped her arm around his waist, and was walking him forward hugged close alongside her as if they were out on the town. She was giggling, as quietly as she could manage, but even so Dipper could still see the occasional person turn, a slight look of confusion on their face at hearing a voice that didn’t match a passerby.
“Dipper, my boy,” she said, as if she was a statesman about to give a speech, “The rest of the evening is ours; We should enjoy ourselves.” The way she said enjoy sent a little happy tingle back to Dipper’s crotch, and he hugged her close to give her a quick kiss on her approximately-cheek. “So where-to first, then?” he said as he finally located her ear and gave her a quick hot nibble on her earlobe.
She muffled a squeal, and pulled his hand towards a lower area of the mall in lieu of pointing. Dipper remembered there had once been a Hoo-Ha’s there, but repeated issues with apparently haunted animatronics stuffing security guards into the puppet suits had led to it’s closure, demolition, and exorcism by an ordained priest. Since then a bookstore and a Yumberjacks had opened, but between them was a-
Dipper paused as Mabel tugged him towards the shop, before grinning like a madman and following her. She must have noticed, because she stopped short, cupping him briefly with a quick fondle, his blood surging southwards as she said breathily into his ear “Last one there is a rotten egg!”
Then she turned, giggling quietly as she ran in the direction of the shop, as Dipper chuckled and ran after his sister.
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Horny Ghosts