How about a blowjob instead?

"Nah," you reply.

Jan is completely taken aback. "Excuse me?! You'd rather come on a towel than have my hand on your dick? You really are a fucking pervert, aren't you, you little..."

"I'm a pervert? Who's the one who just offered to touch her brother's cock?" you ask, smirking.

She turns bright red. "W-well, it's like I said. I just don't want you to get your nasty jizz all over the place. That doesn't make me a pervert."

"So what exactly were you planning?" you counter. "If you're gonna give me a handjob, I still need to cum somewhere."

You can think of a couple easy outs for her, but you're not about to volunteer anything at this point. Your sister's proposal was just too wild to brush off and a part of you really wants to find out what she's thinking. Though another part of you is convinced that this must be some elaborate prank.

"I could... suck on it. A little bit. Just long enough for you to... to shoot in my mouth." She's speaking very quietly and looking down at the floor, but you can see her ears turning red and the corners of her mouth turning up slightly. She's excited. She wants this.

"That would solve the problem, I think," she continues quickly. "There wouldn't be any mess if I swallow it."

You can't believe that your sister, of all people, is offering to suck your cock. You've had fantasies about Jan doing stuff like that, but never dared to dream she had such fantasies of her own.

"And the only reason you're willing to do that," you say, "is so I don't have to stain our towels. Right?"

Jan's head whips up and she locks eyes with you, her expression full of anger. For a moment, you're sure she's going to slap you and storm out, but Jan bites back the anger and draws herself into a more neutral expression.

"Smartass," she murmurs.

Yeah, you probably should've known better than to poke at her like that, even if the story seemed paper-thin. It was downright counterproductive, now that you think about it.

"Alright," you agree. "Deal."

The two of you sit awkwardly in silence for a few minutes, avoiding each other's eyes. Your cock is getting hard and you can't help but imagine what it will feel like when Jan's fingers wrap around it.

"So, uh, when were you planning to do it?" you ask.

She shrugs. "Well, when do you usually jerk off? Is it a regular schedule or just, like, whenever?"

"Uhh, pretty much whenever I get a boner, I guess."

"Oh." She looks away, then looks back. "Do you have one now?"

You nod, not trusting yourself to speak.

She takes a deep breath. "Okay, well, let's take care of that, then. Pull something up on the TV and I'll be right back."

She stands up and walks over to her bedroom. You browse the adult channels and find a nice scene where two women are coyly flirting, obviously working up to some girl-on-girl action. A short time later, your sister comes back with a dark blanket draped around her shoulders. She kneels in front of you and pulls the blanket up over her head, forming a tent.

"You can, um, take your pants off now," she says.

"Yeah, I'll do that, but what the hell is with the blanket?" you ask.

"I didn't want you to be able to see me. So watch the TV, not me."

"Alright, I got it," you say.

You do as she asks and then settle back down, feeling the couch's rough fabric against your bare ass and Jan's hot breath against your bare dick.

"You better appreciate what I'm doing for you, dude," she says, sounding annoyed. "I've never done this before, so I could suck at it. And if you cum too soon or on my face, you're gonna owe me a hell of a lot more than half the rent."

"Yeah, I'll be careful," you say nervously.

Then her fingers gingerly brush the underside of your shaft, causing you to jump.

"Ow! Hey, chill out," she complains.

"Sorry, sorry."

You try your best to stay still as your sister wraps her hand around your cock and starts slowly stroking. You close your eyes and imagine that the women on screen are doing it to you instead. But you soon give up on that. This whole situation is just too outlandish for you to focus on the porn. You've never had a woman go down on you before, and knowing that it's your own sister who is now flicking her tongue against your glans makes the experience so much better.

She swirls her tongue around the head a few times, and then wraps her lips around the tip and gives a gentle suck.

"Mmm. Yeah, that feels great," you moan.

"Shut up, just watch the TV," she mumbles.

"Sorry," you whisper back.

Her hand keeps working the bottom of your shaft, and she takes the top in her mouth, bobbing her head slightly. You feel yourself bump against the back of her throat, but she keeps taking more of you, her nose pressed against your pubes and her hand still pumping the base.

"You're so good at this. I'm already close," you moan.

"Mmf," she grunts, her lips and hand never leaving your cock.

"H-hey, slow down, I'm about to..."

She doesn't slow down. In fact, her pace only quickens, her mouth and hand working faster and harder than before.

"Jan!" you gasp. "I'm cumming!"

Your sister keeps sucking and pumping as your body stiffens and spasms. Then, as your orgasm subsides, you feel her swallow several times, drinking down the cum that you just spurted into her mouth. She draws back slowly, sucking and licking you clean, then leans back and drops the blanket behind her. Her face is flushed and her hair is disheveled, but there's a big smile on her lips. As you sit there, she rushes off to the bathroom and brushes her teeth. Then she returns and flops down next to you.

"Well, now that that's out of the way, you can go take a shower or whatever," she says, as though nothing untoward had happened. "Don't forget, you owe me, dude."