Hot Summer Job/You're so close; just cum, fill her up
You could try to push your luck, but blowing your wad sounds so much sweeter.
The only problem is with the tip nestled inside your sister's delicate entrance your hand has no way of stimulating your swollen glans. You want to cum so badly, and you're not far off. You just need a little something extra.
You adjust your grip then double down on your shaft. Your hand becomes nothing but a blur whipping back and forth, hitting Bree's pussy and your coinpurse in quick succession. Your intense rabbit-like speed shakes and vibrates the end of your cock between Bree's parted lips. Your little sis shifts about some more looking increasingly uncomfortable but the bit of wiggle you're getting inside her tiny hole is just enough to help build your orgasm.
Bree shifts from side to side, looking away just as much as she examines your carefully positioned cock. Her rosy cheeks have blossomed into an even deeper shade of red. Whatever stimulation you're providing her is a new level of embarrassment for her.
"Umm, yeah. You can... stop now," states Bree in a forcibly disinterested tone. "Your dare is over."
"No. I can't," you reply, your balls tightening.
"What? You... can't?" repeats Bree with a heavy breath and a shudder. "Why not?"
"I'm gonna cum."
Your parrot tilts her head. "Cum?"
"Uh-huh," you grunt, your arm beginning to cramp.
"Uh, okay. But..." Bree hums and haws for a moment, looking ashamed. "I'm... getting real wet."
Another grunt gets forced up your esophagus. "Fuck, Bree."
She looks away again. "Sorry. It's just y-"
Instinct forces you forward just as your dick explodes. The head of your cock pushes its way inside your sister's virgin entrance. Her tight pre-tween hole clamps down on the intruder, narrowing your nozzle and pressurizing the hot stream of cum you spray inside her.
"Ah! What's going ON?!"
Bree yelps when another healthy load blasts her insides. She pulls away, but not before you dump a third wad right between her lips. No longer joined, you pump out a few smaller shots onto the front of Bree's swimsuit. She stares down in shock at the milky white ropes that just flew out of your penis.
"Wha...? You... cummed?" she ponders aloud, sounding less than certain. "You cummed inside me?"
Teeth clenched, breath held, you continue to beat your meat, fervently, stubbornly. You force one more shot down your shaft, working enough energy into it to fire most of the way to Bree's chest. This manages to startle her, drawing a gasp.
"Ryyy! This is gross!" complains your little sis, donning a grumpy expression. She pinches the sides of her suit and pulls it away from her body, trying to keep your seeping spunk from touching her skin. For some reason she seems much more concerned with her soiled swimsuit than her cream-pied pussy. "I never said you could cum!"
You take a few quick breaths, recovering from your violent exertion. "Well... you did dare me to jerk off. What'd you think was going to happen?"
"I- I dunno," she admits, pushing herself up. "Not that." Your baby sister sits on the edge of the coffee table, your semen leaking out of her tiny slit and dribbling onto the hard surface beneath her.