Her elder sister, Jane

"Stace! I know you're still in bed! It's time to wake up!"

Stacy grimaced at her elder sister's idea of a joke.

"I'll be home for lunch. Bit of a mess at the lab today, so I've got to shower and change before heading back. Be there in half an hour."

Stacy didn't even have time to check when the message was left when she heard the front door click open.

"Stacy! Don't tell me you're still slee- oh, there you are." Jane put her handbag down on the kitchen counter and looked Stacy up and down. "You've had your shower?"

Stacy nodded mutely. She was still naked and covered herself for decency's sake, but Jane seemed unconcerned as she stripped down on her way to the bathroom.

"Good. I'm filthy, so I'm having a wash before anything else."

Stacy shook herself awake and blushed when she realised that she'd been staring at her sister's hips wiggling as she walked. She couldn't find the words to tell her sister what had happened that morning - if it had happened, she was still questioning herself even now - and she must have spaced out when Jane was undressing.

She looked under the kitchen counter where the cardboard tube had fallen. If it was a dream, she had no idea what parts of it were real and what weren't. If it wasn't a dream... shit. She didn't know what to do.

Stacy was still despondent when Jane came back into the kitchen, stark naked and toweling her hair dry. She walked straight for the fridge and Stacy couldn't help but watch her sister's lithe figure as it bent at the waist to reach the bottom shelf.

Jane rose with what was let of the milk in her hand, tipping it up as she drained the bottle. Stacey's eyes traced the rhythmic ripple of her throat as it swallowed, traced the line of her neck down to the curve of her breasts and below.

She jumped when she realised that Jane was staring back with a look of concern.

"You okay, Stace?"

Stacy felt her eyes jumping from her sister's face back to her body and back to her face again, trying to find a way to hide from both Jane and herself that she'd just been perving on her sister.

Jane's brow furrowed as she strode over.

"You've been a bit out of it since I got home. Has something happened?"

Stacy felt the warmth of Jane's hand against her forehead, saw her worried expression inches from her own face, looked below to the curves of their nude bodies, so close, so... inviting. With a mounting sense of dread, she felt something twitch into life between her legs.

A shiver ran through her body.

Without her thinking, and in spite of her panic when she realised it, Stacy's arms rose up of their own accord, wrapping around Jane's chest and pulling her in for a kiss.

It took Jane some seconds to realise what was happening before breaking free with a startled yelp.

"Stacy! What the fuck!"

Stacy felt her body lurch forward from its seat, propelled as though pulled by a string at her crotch, her hips thrust forward while the rest of her body trailed behind. She landed heavily against Jane, bringing the pair of them down onto the tiles. Jane's head landed with a thump and she cried out in pain and confusion.

Stacy's own hands gripped Jane's shoulders, fumbling their way along her arms as though blind before finally restraining her wrists. Through the horror of feeling her body forcing itself on her sister, Stacy felt something hard and hot emerging from her pussy, already slick with her own juices.

"Jjjaaannne" she managed to moan, as though her mouth and tongue were filled with molasses. "Hhheeelllp~"

Jane shook her head, unfocused eyes widening as she felt something slimy bumping against her inner thighs. With a scream of panic, she hurled Stacy to one side, unable to break the grip on her wrists but gaining enough space between them for her to look, aim and deliver a kick to Stacy's crotch that sent a steel spike of pain into her skull.

As though an electrified wire had been suddenly wrenched out of her body, she felt the foreign control of her body vanish and let go of Jane's hands to double over in pain.

Jane had scampered backwards to the opposite side of the floor, grabbing a ladle from above her and brandishing it.

"Stace, what the fuck is that!?"

Stacy groaned. "I don't... know. I got a... thing in the mail and it attacked me. It attached itself to me... became a part of me."

She could feel her dick's pain subsiding, its arousal withered by the **** as it shrank back into the shape of her clit. She saw Jane's expression of horrified fascination as it did so. Well, at least now she knew.

Jane cautiously edged forwards, ladle still raised. "If I come closer, are you going to attack me again?"

"I don't think so," Stacy said, regaining her breath. "It was making me..." she blushed, "...horny. I think that's the only time it can control me."

Jane's face went bright red at the thought, but she seemed to get a grip on herself as she got closer to inspect Stacy's pussy.

Even recovering, Stacy felt her pulse race at having Jane's face so close to her vagina warned her sister away.

Jane shrank back before motioning to the bedroom with the ladle. "I've got an idea. Get to your bed."

Glad for something to do, Stacy ran to her room and dived under her sheets.

"Close your eyes!" came the yell from the kitchen, and Stacy did so.

What she heard coming towards her was the sound of rustling cloth.

"Okay, stick your left hand out."

Stacy did, and felt something loop loosely over her wrist. The same was done to her other hand and both feet. Without a word of warning, she felt one of the loops suddenly tighten, binding her arm to one of the bedposts.

A shiver ran through her body in immediate reaction to the restraint, her cock erupting painfully from her pussy as it sensed it was in danger. Stacy's body arched, but not before Jane was able to secure the other three restraints. By the time her cock had taken control away from her, Stacy could do nothing but struggle ineffectually against her bonds.

She moaned unintelligibly, unable to form words while her body thrashed around, unable to gain more than an inch of movement in any direction.

Jane had initially stood back with a look of apprehension, but relaxed upon seeing the restraints hold and stepped forward to where Stacy's head lay.

"Stacy, can you hear me?"

"Nnnnnnhhhh~!" Stacy managed, through an incredible exertion of will, to nod her head.

"But you can't talk?"

"Nnnghh!"

"Shit."

Stacy saw Jane look from her face to her cock, which stood rigid above her body and throbbed madly as it sent impulse after impulse like electric shocks through her muscles.

Jane left, and Stacy struggled alone against the bed before she returned with a pair of latex gloves, a glass jar, a sponge and a bottle of baby oil. She put on the gloves with a calculating expression before rubbing the baby oil onto them.

Stacy wanted to shrink back in disgust as she realised what her sister was about to do to her, but her body continued bucking mindlessly.

"I'm sorry to have to do this," Jane said, approaching Stacy with the jar and sponge under one arm. "But I think I can calm you down without kicking you again."

Not like that! Stacy wanted to scream, until the moment she felt something cool and wet stroke her penis.

It was as though she had come under a spell. From the moment Jane stroked her cock, her body fell still and even her mind was stunned at the pleasant sensations emanating from her crotch.

"There," Jane said, as though talking to a small child. "That's better, isn't it?"

Stacy tried to speak again, but was barely able to mumble as she felt her mind drifting away. She closed her eyes as she felt the strokes engulf her cock, rubbing up and down its length with a loving tenderness she didn't know her sister possessed. God, it felt good. Stacy felt peaceful, as though every part of her could shrink away into just her penis and that would be just fine. She just wanted to lie here like this forever.

She twitched when the second hand joined in, the motions becoming more vigorous and igniting a spark inside that made her body arch towards the serpentine hands, their coordinated motions making a squishing noise as they pumped up and down.

Stacy's breathing became shallow and laboured, her body straining again but this time in wild response to the stimulation, twisting against the ropes and grasping at the sheets. A low, animal moan escaped her lips as she felt something building, building inside her and she had barely realised she was about to cum when it happened, her cock erupting with pleasure as her hips shook violently.

It felt like a hundred years later that she remembered her eyes were still closed and opened them to see Jane placing the jar to one side. It was less than a quarter full of white goo, but the gloves and sponge were also inside.

"Stace?" Jane said, putting a hand on one cheek. "You okay?"

Stacy stared at her sister for a few seconds before nodding.

"Can you talk?"

After some time remembering how to move her jaw, Stacy said "Yeah."

"Can I untie you now?"

Stacy closed her eyes. Her pussy was still thrilling with the foreign pleasure, but whatever thing had inhabited her clit seemed satisfied for the time being.

"I think so."

Jane nodded and cautiously untied Stacy's arms and legs. Stacy sat up, massaging the skin where the fabric had chafed.

An awkward silence drew itself out as Stacy tried to think of something to say, her sister staring at her with an expectant intensity.

"Um... thanks," Stacy finally said. "I'm sorry about that."

Jane nodded, but it didn't seem to be what she was waiting for. Instead, she bent down in front of Stacy and gently inserted a finger into her pussy.

"Ah! Jane, what are you-?"

"Shh!" Jane said, her thumb exploring Stacy's clit.

Stacy winced in pain at the overstimulation and Jane relented, stepping back with a thoughtful expression.

"Interesting..."

Stacy goggled at her. "Interesting? This is fucking weird, Jane. What the fuck is happening to me?"

Jane shook her head, but picked up some clothes off the floor and threw them at Stacy.

"I don't know, but it's not safe for you to be like this. You need to come to my lab - I want to analyse the sample I extracted from you and we've got the quarantine facilities to make sure you don't do anything stupid."

Stacy frowned. This was typical of her sister: Logical, analytical and - annoyingly - almost always right. But the idea of leaving the house in her current condition scared her.

"Can't I just stay home?"

Jane frowned. "What? And hope you just get better? I can't study you here and I'd need to keep you locked in at all hours just in case you were triggered by something you saw outside and broke out to **** them. I wouldn't be able to help you then - God only knows what the police, or the government would do with you once they found out."

Stacy shrank back under the admonishment, but even the idea of being locked in the basement seemed appealing to being locked in a lab cage. Then again, if there was anyone she could trust to help her without handing her over to be some government guinea pig, it was her sister.

"Jane, I..."