This would probably never happen in real life, but it's still nice to fantasize about...
I am 19 and I masturbate pretty frequently. My sister is 16 and I'm pretty sure she's never masturbated in her life. Sometimes I fantasize about teaching her how, though...
In my fantasy, I'm sitting on the couch naked jerking as hard as I can, thinking I'm all alone in the house, when suddenly she startles me by walking in and I know I don't have any chance of covering myself up.
'I'm sorry you had to catch me doing this,' I say with my cock still hard.
'Um, that's okay.' she says.
I ask her, 'Do you ever do anything like this?'
'It feels really good. You should try it sometime.'
'Maybe.' She has a kind of amused tone in her voice, like maybe the situation is turning her on a little.
I'm turned on, too. 'Would you like me to show you how to do it?'
She smiles, and says, 'Sure!'
'Okay.' My cock was already hard, but now it's getting even harder. 'Why don't you start by taking off all your clothes?'
She strips pretty quickly leaving her clothes in a pile next to mine. I look over her pale, naked body, her large breasts with big, hard nipples, her unshaven, hairy pussy, and I grin a little bit. She grins and blushes a bit too. 'Why don't you sit down on the couch?' I say.
She sits down, and I get off the couch to go kneel in front of her. I gently spread open her legs, and get my first look at my sister's pussy. I can see that she's already a little bit wet, and I ask, 'Have you ever touched yourself down here?'
'Occasionally, but nothing really serious.'
'Well, it's important to keep it well-lubricated.' She smiles a bit at this. 'I'm gonna start off by rubbing you a bit.' I take my fingers and I rub them around her clit. It swells up a little, and some more juice starts to flow out. I take that juice and I start to rub it on her clit. 'Does that feel good?'
'Good. The clitoris is the biggest source of pleasure that a girl has. You should learn how you like to rub it. Some girls just rub, but others like to stick a few fingers up in their vagina.' I demonstrate with a couple fingers of my own. She's really warm, and very slippery. I thrust in and out a bit to give her an idea of what it feels like, then I pull out my juice-covered fingers and lick them. Her smell and taste are starting to get me really horny.
'Now, I'm going to show you what an orgasm feels like. You've never had one before, have you?' She shakes her head, no. I can tell by how wet she is that she really wants one, though. I lean in closer, and I slowly run my tongue up her slit, savoring the sweet taste. She lets out a little moan as the tip of my tongue flicks against her clit. Then, I start on her for real, furiously licking and sucking at her pussy. I'm so turned on by the way she smells and tastes that I don't take my mouth off her even once. As more juice flows out of her, she starts to tense up and arch her back more and more, closing her eyes and letting out little moans of pleasure that crescendo until they become outright screams of ecstasy. She cums for what seems like a full minute before she settles down some, and I pull myself away from her, leaving her flushed and panting.
'You like it?' I ask, grinning. She just smiles back at me and nods her head.
'Okay, now you try it.' She smiles even wider and eagerly moves her hand down to her pussy. I seat myself next to her on the couch, our warm naked bodies touching as our right hands both work furiously at sending us into new levels of pleasure. I can tell that she's going to masturbate a lot too, and that we're going to be doing it together quite a bit. 'Maybe some other day, I can teach you how to do other things.'
She smiles and says, 'Maybe.'