Some sisterly advice for you

Walking up the stairs, you make a turn for Gwen's room to fix your clothing situation. Let's give her the benefit of the doubt and assume she didn't intentionally give you a pajama that would fully display your fun box. Rummaging through your sister's wardrobe, you find a cute thong and a large, loose shirt to wear instead.

Gwen follows you up a few minutes later with a playful smirk on her face. "I see you found yourself a new outfit, looks... boyish. Why not enjoy your feminine charms while you have 'em?" she asked, and you shook your head in return. "I don't care about those, I just want this to be over."

"Well, that will take time. Until then you may as well stop trying to fight your situation. You know, after you left, dad said that you're an attractive young woman. Hell, he even said he would love to add some mileage to your pussy after you gave him such a good look at it," she lied with ease.

"Eww, gross! He's my father! What's wrong with you?" The thought alone was disgusting, but Gwen didn't seem phased by it. "After all it's not like you two are blood-related anymore, right? Besides, he said he had a vasectomy, so it's not like you would have to deal with any consequences," she added more lies. "Come here for a second." While still talking, she began approaching her bed and pushing you right on it before closing the bedroom door for some more privacy. "Take off your panties."

You're not sure what she's thinking of, but she seems pretty determined, so you're hesitating to follow her instructions. She crouches down near the bed and pulls a small box out from underneath, and after opening it, she withdraws a... glass dildo? And it was attached to some underwear? A strap-on? "Whoa sis, easy there," you chuckle nervously. "I'm not interested in any of that stuff." Gwen however simply shrugs and pulls your panties right off your crotch to expose your cute slit, after which she began to put on the strap-on while still dressed in her jeans and tank top.

"Shush, if you don't want to fuck even after getting an idea of what it may be like, fine. But first I'll show you how good a fake dick can already feel," she proposed and moved to climb on top of you, leaving you **** but to open your legs and make room for her.

"No, really, I don't want to know what it's like. It's kind of... gay, you know?" Gwen however didn't listen to your excuses. She reached for a bottle from her nightstand and lowered it against your pussy, and you suddenly felt a cold liquid squirted into you. "H-hey!"

"Sorry, bro. It's not like a dick, female equipment needs some preparation. And if you don't wanna play along, you'll have to endure some cold lube in there every once in a while. It's cool though," she explained happily while putting the bottle away. "It's special perma-lube. Your playground is gonna stay wet for a day or two with the amount I just squirted in," she winked at you, while you were still recovering from the chilling temperature of the stuff.

Immediately after, you could feel something hard poking against your delicate lady bits and you looked down to see Gwen rubbing the bloated, transparent cockhead against your puffy lips. "Tease 'em a bit first, and then," she lunged forward and penetrated you with the first few inches, breaking right through your hymen.

"Shit!" That hurt like a bitch. You tensed up completely while Gwen was holding still and waiting for your pussy to adjust to the thick intruder. After a brief pause, Gwen began to slowly move back and forth, deliberately stretching your womanhood with her thick appendage. "Slow down, damn, a pussy needs a long time to get going... no wonder that, like, all women bitch about guys rushing it," you complain grumpily while your sister was still rhythmically sliding in and out of you.

It was starting to feel sort of good. A good kind of stretching. The more time passed, the more sensitive did your pussy become. "That's it bro, just relax and let your pretty cunt get used. Lean back and take the dick," she whispered seductively while you eagerly followed her orders. "Just think of how much better it would feel to have your dad's cock inside. A real man. Not like you. You're just a cute teen girl now."

Her thrusting became more eager and demanding, each shove forward hitting just the right places to make you squirm and gasp for air. "I want you to have your first orgasm as girl, Sarah." Hearing those words was enough to unexpectedly send you over the edge, and you felt shuddering, warm waves of pleasure coursing up from the very bottom of your toes up to every single strand of hair on your head, flowing through you until your vision went black.

Phew. Wow. "You okay?" Gwen? You open your eyes with an exhausted smile. "Yeah. That was pretty damn good, thanks." She hands you a glass of water and you eagerly drink it down to quench your thirst. You still felt an itch down there. At least as guy, you were completely drained after a climax. But as girl?... you could already go again.

You awkwardly retrieve your underwear, wave at Gwen and return to your bedroom where you close the door and go right back to masturbating now that you had discovered the forbidden fruit. Your sister probably suspected it, considering she didn't disturb you anymore for the rest of the day. Perhaps she knew that it would all be over by tomorrow. With that in mind, you put the damaged pajama back on and eventually go to sleep after climaxing countless times.

In the meantime...

Gwen had been pretty happy with the progress she made with her new sister, but apparently it hadn't been enough. The sun was down, and there's only... eight or so hours left before the nubile Sarah is turned back into boring John with his awful humor and lack of empathy for women. So much wasted effort. She had even gone through the trouble of dissolving some pretty expensive ovulation **** into the earlier glass of water to help Mother Nature along, but without any seed, there's nothing to take root.

After reading a book for another hour, Gwen left her room to tell her parents good night. The sound of the TV downstairs was extremely loud, so she just followed the noise and spotted her parents in the living room with plenty of booze on the table. Upon walking around the couch, she gasped in surprise while staring at the naked adults - her mother was on top and apparently had been riding dad, but... then they must've fallen asleep halfway through. The scent of vodka, and of pussy was heavy in the air. "Mom, wake up," Gwen poked the mature woman, and she stirred in response. Being a good daughter, she slung an arm around her mom and slowly guided her upstairs and into the master bedroom to sleep it off.

Walking back down, she saw her father still nude, still asleep, his dick mostly flaccid but glistening with remnants of her mother's juices. Once more she got him to wake up as well, and then guided him up the stairs while he clumsily tried to follow along in his partially asleep, drunken state. Right ahead was the master bedroom. Almost there. Gwen stopped when they stood next to her brother's bedroom. It was tempting... without wasting much time, she opened the door into her brother's dark room and nudged her father to enter. "Go on, your wife is waiting," she chuckled, and once he stumbled forward, she closed the door behind him.

Gwen rushed to return to her room and get on her bed to listen to the adjacent room.

Silence.

She waited another minute... a few minutes... nothing was happening. Aw well... it had been worth a try. John was kind of okay as a guy anyway. He was able to help carrying the groceries, took care of spiders, he had his uses. Still, somewhat unhappy with the outcome, Gwen undressed and switched into her own pajama. She brushed her hair, took off her socks, and climbed back into bed.

Thud, thud, thud

Huh? She leaned towards the wall and listened closely.

Thud, thud, thud the heavy noise continued, and she recognized it as the sound of John's bed, the headboard must be slamming against the wall!
"Nnnh, ahnn," a female, girly voice suddenly moaned in tune to every hard knock. It was happening! Gwen happily listened to the private concert she was treated with, the sounds playing in tune to one another. "That's it, dad. Get deep in there. It's only going to take a little bit of sperm and I'll get to keep my new sister." The constant thudding continued, mingled with those cute female gasps and moans that were barely even audible.

Until finally, a male voice groaned loudly and strongly, followed by complete silence.