Nathan followed his sister through the tall grass, his legs constantly tickled beneath the hem of his shorts. The sun was near its peak, and the humidity increased the closer they got to the river. His  sister walked a distance ahead of him, occasionally turning to encourage him farther, her black-and- white tail bushy in the summer breeze as she whipped back around to keep ahead.

Their parents had married some time ago, and they'd been glued to one another ever since. No matter what they did or where they went, Suzanne would take charge and Nate would follow. She was fearless,  her fur sleek and soft, her face strong and set whenever she was unsure. She wasn't anything like Nate.

Nate was shy. His fur was often shaggy and dull for lack of brushing, his black-and-white tail held  low to drag through the grass. He had few friends, and spent little time outside of his bedroom. His  mother's remarriage eventually became a good for him. Suzanne was an incredible role model, and he  now found himself striving to impress her. He was still incredibly awkward, though, and his social  life away from his sister was lacking to say the least.

They were near the river now, and Nathan could hear the telltale sounds of its constant motion.  Suzanne gestured toward him, assured him that it wasn't far now. She had been here before with her  father, and the cave was just a few minutes' walk from where they often swam. What she didn't say was  that her father, upon her discovering the small cave entrance, had told her to stay well away. The  smell of the place was off, and there was nothing good to be had down there. She hadn't listened,  though. She was certain she had found something incredible, and she wanted to see what was inside.

Nate called to her and asked for a break. The backpack he carried was getting heavy, and he was  sweating profusely. The trees formed a canopy here, and their leaves held back an incredible amount  of  moisture. Suzanne backtracked to her brother. He was bent forward, hands on his knees, panting  from  the  humidity. She reached toward him and relieved the weight from his shoulders. In a single,  quick  motion, she swung the backpack around herself, and continued on. Nate took another deep breath  and  followed.

They reached the cave mouth in short time. Suzanne was unphased by the weight she carried. She was  sweating, though, when she pulled the bag from her back and opened it. As Nate caught up, she handed  him a flashlight, and equipped one of her own. Up again the backpack went, and she stooped down at  the entrance. The air that crept from the rocky hollow was cool but stale. Suzanne didn't smell  anything off as her father had. The beam from her flashlight revealed little aside from smooth rock  and dust motes. She stayed hunched down as she steadily made her way inside.

After a while the cave opened up, and the two found that they could walk comfortably. Suzanne pushed  on, and she swept the beam from her light across the floor. Nate found himself constantly looking  back. The light of the entrance had been lost some time ago, and the darkness seemed to press in  toward them. While the dampness on the walls and floor yielded no humidity, it was hardly comfortable  down there. Nate found himself walking just a bit closer to his sister.

Finally the cavern opened up. The ceiling went high, and the walls went wide. The walls were smooth,  and the ceiling and floor were bare. Suzanne hadn't noticed, but Nate found it odd. Shouldn't there be spiky things all over the place? Suzanne's light never held still, and she swung it over the ceiling and across the far wall. Nate's moved far more slowly. While he could see the color of the rock, he found it difficult to make out any detail. Wherever his beam traveled, the surface of the  cave seemed oddly reflective. It made him uneasy, and as Suzanne moved to step into the cavern he heard her foot squelch as it came down. She made a noise of disgust. This stuff must be everywhere  down here. She tried to step back, and came up short. The colorless stuff, only a couple inches thick at most, refused to release her foot.

She was stuck fast.

Suzanne tried again to pry her foot from the floor. She gave it everything she had and then some, but the most she managed to do was slide back a few inches. Nate reached for her, tried to brace her and  help give her some leverage, but as his hand neared her a tremendous sound rolled out from the cavern. It was deep, guttural and wet, like a pocket of gas forcing its way up through mud, amplified  a hundred fold on the rocks and through the passage. Nate froze and looked into the deep black, his light beam quivered as he tried to see what made the noise.

"Suzy?"

There was movement, quick as lightning, and Suzanne screamed as she was pulled bodily some several  feet farther inside. Nate didn't have time to register the sight of his sister being pulled to the  floor as his own balance was made forfeit. In an instant something cool and wet pooled around and up his ankle, and before he could even flinch he was on his back. His flashlight clattered to the floor  as he was also dragged into the expansive room.

"SUZY!"

It was dark here, and it was wet. There was no friction at all. All he could feel was the damp stuff on the floor as he squirmed to gain purchase that never came. 

"N-Nate! Nate, don't struggle!"

Nate looked up at his sister, the fur on his face already streaked with tears. She had been dragged  farther in than he had. She was positioned in profile to him as she faced the center of the room, but her face was turned. There was fear there, in her normally set expression, but it was under control. 

"T-try not to move, okay?"

His voice caught in his throat, so he nodded his head instead. Immediately after he made the shallow motion, the translucent stuff rolled up over his neck. He went stiff, eyes wide. The cold goop held  him there on his back as it encapsulated his body, but it slowed to less than a crawl as he held  still. Suzanne, however, hadn't heeded her own advice. The stuff rolled up underneath her as he watched and slowly raised her to a near-standing position. He saw as she fidgeted, her arms down near her waist.

The stuff had permeated her clothes. She could feed it slide against her skin, and she wondered how  it managed to keep such a firm hold on anything. But it was firm, all right, down around her knees,  up her thighs.... It had formed a sort of cushion around her body, suspended nearly as high as she  was tall. She could virtually see through it when she looked down, the colors of her fur and shorts  just visible in the scattered beams of the abandoned flashlights. Still the form grew around her,  amassed from unknown depths of the cave, and the noise rumbled from the deep once again as she felt  the cool, viscous slime press against her lower regions.

Nate watched as Suzanne's near-calm complexion drained in a heartbeat, saw her squirm again,  virtually spasm in the goop. She cried out.

"N-Nate, its-"

Her eyes bulged.

"Oh god, it's-"

She felt as, despite her best effort, her tensed back, her held breath, it ceased to crawl over her lower lips, then parted them and poured inside.

Nate saw the bulbous form around his sister sort of fold in on her, and he tried with all his might to stand. The stuff woke up around him and played across his body like the coldest, most  inappropriate hug. The goop pressed in on him, held him fast. His eyes went wide as his sister's belly began to swell before him, and then he realized what it was doing to her because it had found  its way inside of him as well.

The thing felt cool pressed against Suzanne's skin, but it was ice inside of her, and her voice ceased to obey. She wanted to scream, to cry out to Nate, but she could only watch and feel as the stuff spread her wide and forced its way into her, slowly at first, but steadily faster as time went on. She saw her belly begin to bloat outward, felt the hem of her shirt ride up it as her navel  peeked out underneath. Her face burned. There was no pain, but a sort of intense pressure. It was  alien, and it sort of pulsed as it pushed farther into her. She blushed deeper as this icy creature burrowed further, filled her up, and exposed her to her brother. She hated this, hated the fact that he had to see her this way. Then she heard him moan.

The thing felt cool pressed against Nate's skin, but it was fire inside of him, and his voice refused to die down. The goop pressed against him, spread him wide, and forced its way inside. It was strange but familiar, like he'd been meant for this. Its pulsating form expanded inside of him, pressed against his innards with a deep and immense pressure. Its constant, aggressive thrusts sent shivers up his spine. It continued to rush into him, and he cried out once more as he felt the pressure build. He felt his belly spread mostly sideways as gravity pulled at it, but all of his attention was on the single,  finite point beneath his tail where the vast heat and he became one infinite, burning being as it never ceased to drive itself into him. 

Suzanne watched, stunned, as her brother groaned at the roof of the cave, his voice amplified a  hundred times by the rock and passage behind her. Her voice still refused rise as her eyes focused on Nate, who writhed on his back as his stomach became a gelatinous, quivering mass that touched  the floor on either side of him. Her face still burned. He was actually enjoying this. She looked down. She could no longer make out her legs beneath her swollen, bulbous belly. Her shirt had ridden all the way up to her breasts now, which were also  beginning to swell. She felt so full, but so empty. The stuff was so cold inside of her. The pressure was  immense now, and still she grew. 

Nate finally looked up, and his voice grew louder as he saw his sister.

"OooOouhhgh, SuuoOoUzy!"

He blushed as well at what he saw. Suzy was still in profile, her bare, massive stomach bloated and taut-looking. Then he saw her breasts. 

He had walked in on her once while she was changing. She had just gotten home from classes, and she had left her door slightly ajar. She hadn't noticed him. He had quietly and immediately made his way to his bedroom, and locked the door. He had always looked up to her, always admired her. She was a college freshmen then, but he was only in high school, and his face burned with shame as every night afterward he pawed off to the mental image of her chest. 

They were different now, though. Bigger. Rounder. Her nipples virtually pierced through the fabric of  her bunched-up shirt as it rose slowly higher on her form. Her belly was enormous now, nearly three feet wide, and looked bloated as all hell. She looked as though she were absurdly pregnant. She had tight, bloated-looking love handles that wrapped around her and allowed her stomach to sort  of mesh with the rest of her physique. Her arms and legs were spread wide, held firm by the slime around her, and her tail was splayed out behind in an apparent attempt at balance. Where she was moderately clothed before, her black- and-white patterning was now mostly visible if considerably stretched. Her shorts had slid to her ankles some time before, and all that was left were her slim panties, a dark green, but those were also riding low.

Nate could feel it. He needed it. His face burned constantly now, but there was nothing he could do -  his belly was massive now, too. His hands played over it in an attempt to reach his junk. It had give to it, as it wasn't bloated like his sister's. It sat about three feet higher than it once did,  and had spread across the floor about a foot on either side. The creature moved within him, its thrashing visible as his skin pulsed with it. Its form pressed out against his gut in places, and formed little bulges that disappeared quickly as the stuff retreated. His belly was so soft, his fur and the goop warm between his fingers, and he could feel his thighs and arms were soft as well. He could even feel it in his face, under his chin and around his neck, and as he reached again across his vast, jiggling  expanse of a gut, he could see his chest had considerably softened as well. But his sizable moobs were nothing compared to Suzanne's massive, bloated bosom, now bare as her shirt had been spirited away over her head. 

Suzanne gasped for breath. Her massive chest and ridiculous stomach made breathing difficult. Her feet touched the floor, but her stomach touched even more of it. It must have been five or six feet  in diameter now, stretched and creaking. It pushed her breasts, now bigger than her head, up against  her chin, and her head was forced to tilt back. The slime's assault was beginning to slow, the strongest pressure now in her chest as the creature engulfed and forced its way into her nipples. She shivered at the violation. She could  just about scoot herself in a sort of exaggerated waddle, her arms stretched out and waving about for  balance. As she turned herself, she saw the last of the goop creep its way from the floor up underneath her brother's belly.

Her own belly was huge. She couldn't reach around it if she tried, and it pulled her forward into an  extreme hunch. Her breasts were like pillows then, cushioning her face against her bloated sphere of  a stomach as she tried to turn her head to get a better view. She could feel the stuff moving around inside of her, felt the slick and damp residue it had left on her fur, and it made her feel nauseous. It was nothing, though, absolutely nothing compared to what Nate must have felt. 

The once lithe, awkward boy she had lead about was massive  now, his clothes in tatters, and he gasped for breath as he lay on his back. His face was incredibly round and soft, cushioned by several chins and a pair of massive, jiggling man breasts that folded around the lower half of his face. His stomach was as gargantuan and oblate as an over-inflated  waterbed. His thighs were like tree trunks, tapered to rolls that formed around  his wiggling feet, and between those massive thighs was a pair of bloated, drooping balls bigger than  her breasts - and they were quivering. His entire form, in fact, was quivering. Where she could feel  the stuff move about inside of her, she could actually SEE it move inside of him. His gravid orbs, his gelatinous stomach, his arms, legs, and even his face all rippled as the thing settled in.

Her face burned. Even though she couldn't see his shaft, she knew it must have been hard down there, buried beneath his mass, because he was still moaning. 

Suzanne tried to waddle to him, to turn, to do more than just rock back and forth. Her form was incredibly heavy, and sloshed with every movement. Her bosom was in the way. She tried to look at him, but she had to fight for it, her chin constantly and persistently pressed upward by the gravid, bloated orbs protruding from her chest. Her massive belly wobbled and swayed as it dragged on the floor, her legs spread wide to accommodate it. She managed to shift her mass and face her brother.

“N... Nate?”

She saw his arms struggling across his vast expanse of a gut.

“Are you okay?”

She saw his sausage fingers spread wide, playing into the gelatinous, jiggling flesh of his stomach.

“I... I can't reach....”

Her face burned still, and she realized what he was doing.

“I... can't reach it, Suzy....” 

His eyes were on her massive breasts, his hands pointlessly wandering to find his tender bits. She knew they'd never get there, where they'd been going since he first looked at her bare chest. She had seen him that day, and knew why he'd retreated to his room. She knew what he'd been doing. Deep down, in some dark corner of herself, she kept a strong desire for him to finally find the courage to retreat into her room instead. She'd always imagined that shy boy she knew finally taking charge in the dark of night. She'd imagined waking to that lithe form held over her, swaying back and forth as it drove itself home. 

That lithe body was gone, replaced by the enormous black-and-white mass before her, now obviously reaching to relieve itself by the sight of her swollen tits. 

It was close enough. 

It was as though a switch had been thrown. Ice turned to fire, and her strained panties, clung low to her form for dear life, were soaked through in moments. She looked down at him, chin and face buried in her encroaching chest, and saw that massive, quivering boy she'd lead about for so long struggling to get off. Her hands wandered upward, cupped a breast each, fingers dug into the taut and swollen flesh there. Her nipples positively oozed the stuff inside her, hot and sticky. 

“Is this what you want, Nate?”

His eyes were wide, his breathing laborious. He panted heavily. He was burning up now and couldn't speak, so he whined instead. The low, persistent sound grew in his chest to an insistent wail, and he nodded, an energetic motion that sent his gelatinous, quivering face wobbling. 

Her legs were spread wide by her bloated stomach, but it didn't stop her from moving now. She scuffled along, her pendulous gut dragging on the smooth rock floor, her hands wrapped around the sides of her taut tummy, trying in vain to hold it up. Her eyes were locked in his, her face flushed and full, and sweat ran down her brow. She grew closer to her brother, and his panting tongue found its way to the side, his mouth agape and waiting for her.

She was nearly there, but she couldn't kneel to save her life. Luckily the situation resolved itself, and her trembling, exhausted legs gave out. She rolled forward down her gut, her massive breasts cushioning her head against the ground. Her bloated stomach was compressed beneath her, and kept her a few feet off the ground. Her airbag-bosom had more than enough reach to it, though, and her face was still compressed in its depths, her nipples jutted out like spears before her.

She could see his enormous, jiggling belly over her horizon of cleavage, could even make out his moobs, but she didn't see his face until his wandering hands found their way to her. Her breasts had been sensitive in her own grasp, but they stung under his, the electric jolt producing copious amounts of the goop which ran down her brother's chins as his lips met her flesh. She cried out, her voice nearly deafening compared to Nate's muffled groans. His jaw worked overtime, his tongue ran across her burning skin, his teeth gently caressing more slime from within her. She could feel him swallowing the stuff. 

Her head was spinning. The whole room, in fact, turned around her. Nate aggressively pulled from her teat, her jelly-milk stuck to his facial fur. He was hungry. He was feeding. Her head swam in a cocktail of hormones, and her hand found its way to the back of his head, and she pulled him in tight. His mouth worked with renewed vigor. He was so hungry. She looked over what she could see of him past her breasts, his gargantuan gut writhing from the creature within. He had somehow managed to roll onto his side, or at least partway there to better reach her. 

She was so dizzy, seeing much but recognizing little. He was her hungry, growing boy. She was feeding him, for his own good, to sate his enormous appetite. Pleasure coursed through her, nearly unbearable under his squeezing hands and lapping tongue. But there was something new, now. She was a mother nurturing her kit. 

Something changed within Nate. He began to shake violently,  and the shudders reached every corner of his mass. He pulled back from his sister, saliva and slime strung between her nipple and his lips. He could feel his form rock with the motion, his vision shaky as his face rippled.

“S... Suzy?”

She saw his body moving, writhing on the ground before her. She heard his worried cries. Her head pounded, and the world was skewed. He was so, so hungry. She put her hands on either side of his face, both of them sunk deep into a soft, quivering cheek.

“Shhhhhhhhhh....”

She pulled his mushy face to hers, locked lips with him, and slid her tongue into his groaning mouth.

Nate's heart fluttered, his face a deep, burning red. Her tongue played on his, ran across his teeth, and coaxed his own into rising. He hesitated for a mere moment, then jabbed his tongue forward into her mouth. She moaned, then, as he tried to imitate what she had done, but with jagged motions, his breath pulled in through his nose sharp and hot. His eyelids were shut tight, and his hands reached behind her head to pull her into him. He didn't want this to end.

The thing inside him thrashed still through these developments, and as Nate's grip on his sister's head formed a tight and secure seal between them, the creature's sporadic movements died, it gathered itself up into one mass, and it thrust upward with all its strength.

The violent and sudden invasion pulled Suzanne back into the real world. Her eyes snapped open as the thing twisted and burrowed past her lips and down her throat with speed. She tried to pull her head back, to push against Nate's, but he held her fast, his eyes rolled up into his head. His tongue had died in her mouth, limp and motionless as the goop forced it down against her own. She tried patting his head, smacking it repeatedly to wake him up, to get him off of her. She could feel the thing pushing against her insides, forcing her ever wider, swelling her even more than she already had.

She cried into Nate's mouth, again and again, as her hands were swallowed by her blimping arms. She could feel the smooth rock against her toes as her feet were immobilized, spread even farther apart by her massive, swelling legs and thighs. Her stomach crept across the ground, wider and wider as it was rapidly filled. Even her ass and back were filling up, pushing their limits in defiance of the open space around her. Her insistent cries died in a single, drawn-out low moan, muffled my Nate's muzzle.

Nate finally came to, his lithe frame slick with sweat. He tingled all over, his body still trembling from the thing it had contained. His head ached slightly, and his knees shook horribly, but he managed to lift himself from the hard ground and lean against the massive, warm, pulsating thing in front of him. 

It took him a moment. The light was dim, and his vision was a bit blurry, but he soon realized what the thing was. He took a step back, then another. Finally he turned back, leaned down and picked up one of the flashlights, still covered in slime. He cast the beam across the massive form in the center of the cavern.

The thing that had been Suzanne was something else, now. It was massive, damp and bloated-looking. There were mounds of near-spherical flesh where its arms and legs should have been, its hands forced far to either side of its head, swollen fingers spread apart by one another and uncomfortably splayed, nearly swallowed by its bloated flesh. Its legs and feet were in a similar situation, toes large and shining like over-ripe grapes. 

It was still lying on its stomach, if you could call it that. Its body was enormous and oblate, its form noticeably wider than it was tall, its black-and-white hide creaking audibly. Its head was sunken deep into itself, the skin of which was stretched horridly, its muzzle and eyelids forced wide open by its taut and groaning hide. Now and then the slime creature's movements inside this atrocity, unnoticeable through its horribly tortured flesh, caused a slight up-wash of fluids to splash from its mouth and run down its chin. Its bosom....

“... Suzy?”

… Its breasts were incredible. The gargantuan, gravid things rested on the rock before her, each easily three feet across and as many high. Its nipples were stretched, and a small fountain of fluid drained constantly from each. Nate felt himself grow harder than he ever had, larger than he remembered, and his balls ached horribly. 

He could reach now.

His hands played over his sensitive shaft, his balls hung near to his knees, still just as large as the creature had filled them. The ache was so horrible, so deep.... He took a wide waddle of a step toward the thing, its eyes on him now, his balls singing to him of an impending release.

Suzanne's head was drowning, the room spun so fast around her she could hardly tell what she saw. But she did recognize one thing. She had become trapped, completely and undeniably a prisoner of her own body, of the thing inside it. She couldn't move a muscle. Her hide cried out to her, the pain reduced to a numb and irritable buzz. She had been wrong. Nate wasn't her kit. Not even close. He was her mate, and this thing that filled her to the brim was theirs. They had seeded it, and it had grown inside her, and it was so, so hungry. 

It all made sense now. The hunger wasn't hers, it came from within. As she looked at her brother's cock and the juices ran down her chin, it was because this thing wanted it, not her. It all made sense. When the fire inside nearly burned her alive with lust, it wasn't because she wanted to fuck him.

And she DID want to fuck him. More than anything.

But that's now why she needed it. That meager shaft before her, pumped furiously as Nate drew near, those balls big enough to smother her, those weren't for her, her twisted reason said. Those were for her growing kit. So big now, so nearly ready. So when Nate's legs came to rest on either side of her breasts, when his knees pressed down into their sensitive flesh, her swimming head had figured it all out. She pushed her tongue out, let it fall down her chin, and waited for him.

Nate's dick burned now, as it slid nearly friction-less between his sister's massive tits. The goop worked wonders as a lubricant, the tight orbs smothering his cock between them. His panting breath combined with the constant and repetitive squelch of his thrusts. His hands played on her nipples behind him, his huge balls rested comfortably on their bloated, squishy tit-perch. He fucked them with everything he had and more, consumed by every desire he had ever felt for them. 

Finally he worked past his shame, his burning face turned upward, and he looked at her. Her tongue was there, her maw open and inviting. He leaned forward, put his hands on her face, and kissed her again, still thrusting into her chest. His tongue played over hers, still lying motionless at the bottom of her mouth. He pulled back, and ceased his assault on her boobs. His face burned so much.

“Y... you want this?”

She just sat there, unable to do anything else, her eyes locked in his. 

“I... in your mouth...?”

Still nothing. He hesitated, unsure of what to do now, his balls' song growing ever louder. Was it really what she wanted? His balls twinged, the pain returning now that his thrusts had ceased. Another thought crossed his mind, his head foggy with denial and need. Who was going to stop him? He had her all to himself now. She was finally his. He stood, legs still on either side of her tits, and slotted his member against her open and waiting lips. No warning, no more words.

He thrust with all he had. Down and down he went, as he fucked her face, the fluid splashing against his hips and groin. Her throat took up a rhythmic song as he speared into it, her lips and nose pounded by his thrusts. 

“Uuurk, uuulk, urrrp, uuuuuurrrlpk....”

His balls still rested on her tits, higher up than before, and as neared his orgasm they tightened in a harsh and unforgiving grip. He nearly screamed, his cry directed to the roof of the cave, his balls clenching like nothing he'd ever felt. He shot rope after rope of spunky skunk seed deep into his sister's muzzle, his hands held tight to her head. 

Suzanne felt it, its taste overwhelming her senses. Finally she had it, had HIM. He was all hers, and nothing was going to stop her from guzzling his cock. She came, then, a veritable torrent of ooze and slime, and she felt the pressure diminish considerably. 

***

Suzy was on her back, the sheets tight in her hands. He had woken her up, peeled back her night shirt and panties, his cock at the ready. He pressed into her now, her legs spread wide beside him, a hand over her mouth to stifle her moans as he tried to choke back his own. They didn't want to wake their parents. 

His slim form writhed into her, back and forth, deep and fast. His massive balls were sat on the edge of the bed, finally free from his custom-fit underwear. She remembered now how, when he tried them on, she had sneaked into his changing booth and ridden him like a fucking racehorse. She moaned into has hand as the other squeezed and massaged one of her huge breasts, nearly three times the size they were before. She remembered how, as she first tried on one of her new bras, he had sneaked into her changing room and slotted himself between them, her mouth open and inviting.

He finally came deep inside her, his thrusts sporadic and hard. He panted as he finished, and pulled out of her, sliding up his underwear over his still-hard cock.

“Good night, Suzy....”

As he closed the door behind him, she knew it wouldn't be. Not for sleep, anyway. He'd still wake up, hard as a rock, and come to her three or four times more tonight, each fucking faster and more brutal than the last, and when she finally awoke the next morning, she'd ache so badly she wouldn't be able to walk normally.