I'm going to try this in the first person. Hope it's ok.
To your surprise, your mom and your older sister, Claire walk in to your bedroom and close the door.
"I hear you've been a naughty boy", your mom says with a slight smile on her face. "You've been peeking while your sister and I have been taking showers".
Feeling embarrassed at having been caught, you start to protest but your mom raises her hand to stop you.
"Don't try to deny it". She says. "I know you have. I'm not angry, just disappointed that you couldn't control yourself. I guess it's understandable with two very attractive females living in the same house. But still, you have tried harder. Obviously, you need to be punished for this. And unless you want your dad to find out that you've spying on us, you will let us use your new toy on you for three days each".
You look at your mom and sister. You have a really bad feeling that they'll do something unpleasant if you agree, but your dad will definitely do something unpleasant to you if he finds out you've been perving on your own mother and sister. So, reluctantly, you decide to agree.
You let out a forlorn sigh and reply; "Ok mom. I'll do it".
Your mom smiles and looks at Claire as she says; "See? I told you he was more scared of his dad than he was of us. Now, let's see what we've got here, go sit on your bed".
You nervously get off the chair in front of your laptop desk and go and sit on your bed. Your mom sits down and stares for a couple of minutes before pressing the F1 button. A list of questions appears on the screen and she starts typing.
Nervously, you ask; "Erm...wh...what are you gonna do with me"?
Your mom smiles as she types and replies; "Something that'll teach you a lesson about spying on your sister and I while we're naked. I'm not going to tell you what you'll be spending the next three days as. I want it to be a surprise. How long is the transformation to last? Hmm, there's an option here to make it permanent". Your mom looks at you with a grin and then looks back at the screen. "I don't think we need to that. Yet. Three days should be long enough. Next question: Do you require a clone? For three days, yes. Don't want you being missed".
Your mom answers the other questions in silence and you watch with increasing fear. You have no idea what's she's going to do with you, but you're pretty sure you won't like it. Finally, your mom gets to the last section. For the question asking when the transformation is to happen, she types in 'fifteen seconds'. Then she presses the F2 button to accept the transformation. A message then appears on the screen that reads; "Commencing Transformation Scan. Please Wait". A couple of seconds later, a blue light fills the room. At this point, you seriously think about running, but your mom and Claire are both between you and the door. Despite being stronger than each of them on their own, you don't think you could get past them both.
The blue light fades after a minute and then another message appears on the screen that reads; "Transformation Scan Complete. Subject Identified And Processed. Transformation Will Occur In Fifteen Seconds. Press F10 To Cancel". Your mom sits back in the chair and smiles as she turns it to face you.
"There", she says. "All done. I'm sure the next three days won't be much fun for you. They'll certainly be...interesting...for me though".
"What've you done"? You ask, feeling your fear grow.
Your mom giggles as she slowly licks her top lip and replies; "I told you. I want it to be a surprise. Don't worry, it won't hurt. I wouldn't do that to you".
You look at the screen just as the countdown clock reaches zero. You hear three loud beeps from the emitter and the next thing you know, your whole body slowly dissolves into a white mist. You slowly float toward your mom and then your vision goes black. You feel yourself touch her, and then it gets really confusing. You hear a muffled groan from high above you, and you suddenly feel something warm and velvety smooth gently surrounding you. Then you feel on odd sensation. It feels as though you're body is stretching out and sort of...melting. After a few minutes, your body seems to solidify. You feel something against your cheeks. Something soft, warm and smooth. You try to move, but you find that you're completely paralysed, which sends you to the verge of panic. You have no idea what's she done to you, but since you were floating toward your mom before your vision went, you guess that you're now either an item of her clothing, or part of her body. The latter is something you find particularly unpleasant.
You feel movement, followed a few seconds later by what sounds like a zip being pulled down. Then you see a bright light and you find yourself looking in the full length mirror on the back of your bedroom door. Your mom is standing with her jeans and panties pulled down to her knees, with her hands on her hips and big smile on her face. You can't see yourself in the reflection, but you appear be at the level of her vagina, which is staring back at you. Slowly, the horrifying realization of your mom has done to you sinks in. The reflection of your mom's completely shaven genitals, is your reflection. You're mom has turned you into her vagina.
As you stare, horrified into the mirror, you notice Claire standing just behind and to the right of your mom. She looks as shocked as you are.
"Oh my God", she says. "You didn't"? She then starts laughing. "You turned Jeff into your pussy? Mom, that's sick. I like it though".
Your mom shrugs as she grins down at you. "Well", she replies. "He wanted to see our pussies. I figured it would be a fitting punishment to let him find out what it's like to BE a pussy".
Her left hand comes down, and you see her gently pat her pubic mound, which you feel on the top of your head.
"This is why I wanted it to be a surprise for you". She says. "I imagine it's quite a shock to find that you're own mother has turned you into her vagina, but if it's ok for you to take a sneaky peek, then I think it's ok for me to do this".
"What's it feel like"? Claire asks as she stares at you in amazement.
"Doesn't really feel any different". Your mom replies. "I could feel him go inside me, and I could feel his body transform into my vagina, which was actually quite pleasurable. Now he feels like my real one. It's KNOWING that he's down there, fully aware of what I've done to him that makes it so enjoyable. And I am enjoying it. And I gotta say, he looks pretty down there".
They both start laughing and then your mom takes hold of the side of her panties.
"Well", she says with a contented sigh. "I can't stand here looking at my new lady parts all day. I've got things to do. Look on the bright side kid, at least you're part of a hot woman's body. Even if she is your mother. I hope you enjoy the next three days between my thighs, I certainly will".
She starts to pull up her panties, and Claire asks;
"Wait, what about the clone?"
Your mom replies. "According to the transformation thingy, the clone will become active in five minutes. So nobody but us will know where the real Jeff is".
She then winks at you as she pulls her panties all the way up, pressing what used to be your face against the fabric and plunging you into darkness again. Then it gets a little tighter as she pulls up her jeans. Again, you hear the sound of the zip being pulled up, and then you hear muffled voices and feel a shaking as your mom starts laughing. Then your mom starts walking.
You're in shock. You can't believe that your own mother has turned you into her vagina. You find yourself wishing that you'd never given in temptation and spied on her, but it's too late now. For the next three days, your trapped between your mom's thighs. And you just know that it's going to be the longest three days of your life.
"I can't believe you did that". Claire says with a slight chuckle. "Turned your own son into your pussy. Wait...you're not gonna...y'know...with dad. Are you?"
Your mom gives Claire a mischievous grin as she shrugs her shoulders. "We'll see what happens when your dad gets home tomorrow". She replies. "I am gonna give him a good playing with tonight though. Or should that be her? Considering he's part of woman of now. And I've got to say, I don't feel even a little bit guilty about doing this. I'm really enjoying it. And I'm already considering keeping the clone as my son, and letting Jeff be our...toy".
Claire looks at her mom in shock, but says nothing.
"Just wait until it's your turn with him". Your mom says. "You'll understand. What're you gonna be doing with him?"
Claire doesn't answer for a few seconds. Then she grins and says. "I haven't decided yet. I'm thinking of either turning him into a dildo or a vibrator, or just shrinking him and slipping him inside me. Well he's already seen the outside of our pussies, I figure he should get a good look at the inside".
Your mom fakes a shocked expression. "CLAIRE!" She says with a gasp. "You're a very naughty girl. You take after your mom".
They both start laughing again as they head for the stairs and go down to the kitchen.