She teases you and makes you go finish up

"I mean, I know you're an eighteen-year-old boy, but seriously, can you not go ten minutes without jerking off?" At least she didn't seem furious about it. You risked a glance and peeked up at her face. A smirk covered it.

"I gotta say, I never realized how 'wet' you got!" This confused you, and caused you to tilt your head and give her a look.

"Look at your pants! This is why a little slut like you should never wear khakis! If you're drip through your panties all day, at least wear something dark!" she teased.

Looking down, you realized what she meant. A dark precum stain marked the front of your pants. Your little guy had leaked so much, you had soaked right through your khakis. With no underwear to soak it up, you'd had no chance. And it certainly hadn't helped when your sister's giant tits were pressed up against you. Those D-cups had always been able to get you going; you had spent a fair amount of time investigating her bras, and were used to sneaking glances when you thought no one was looking, but having them right against you was another deal. Still, it was worth it – your sister seemed not to have realized the full extent of your perviness yet. And even this level of embarrassment was better than the alternative.

"You know, James, girls don't normally get that 'wet!' Were you thinking about cute boys?" she teased. "If you want, I can take you shopping to pick up some cute lingerie to impress your first big college stud!" The image of you getting to see her try on lingerie definitely got you going, and you felt a rush, unable to contain the ever-so-slight moan that slipped your lips, accompanied by an involuntary little pelvic thrust and the usual rush of liquid.

You had thought it was pretty subtle, but Molly's eyes narrowed slightly and a difficult-to-read look crossed her face. She didn't say anything, though.

"Alright. I've still got to get dressed, and you can't watch, you horny little thing! So go jerk off quickly in the bathroom so we can go!"

You were shocked that your sister would be so brazen. Was she serious?

"Go! We don't have all day, and if you hope to get the big O, you'll have to be quick! Unless you want to spend the whole ride there frustrated as a virgin on her first date!" She grabbed your shoulders, turned you right round, and slapped your ass, sparking a yelp from you while she giggled.

As awkward and embarrassing as that whole exchange was, your sister was right. You had just been interrupted masturbating, and had never quite reached "le petit mort." With all that had happened this morning, you were far hornier than normal. A quick jerk and spurt into the toilet would probably be for the best.