Sibling Rivalry

Mari took her coat off as she entered the house and hung it up on the rack next to the door. Her mother was sitting on the sofa in the living room, drinking cocoa and watching a true crime show on tv
"Hey, mom!"
"Oh, hello sweetie! Did you find the toys you were looking for at the store?"
"They didn't have them, but they said they can special order them for me, so I'll drive back there tomorrow to pick them up. Where are Sunny and Haru?"
Her mother gave her an odd look, then glanced at the door to their bedroom upstairs
It took Mari a minute to put it together. "They're STILL in there!?" Her mom nodded and took another sip of her drink
Bewildered (and curious, despite herself), Mari went up the stairs and pressed her ear to the bedroom door. There were obscenely lewd slurping and moaning noises coming from the other side
She opened the door, and was greeted by the sight of Sunny and Haru in a sixty-nine position on her bed (that was going to be a bitch to clean...), both their faces screwed up into some combination of lust and stubborn determination. They hadn't even noticed Mari had come in until she spoke
"You two are still going at it? You've been doing this since I left, like, an hour ago..." Mari said, in utter disbelief
Haru brought her mouth off of Sunny's cock with a wet popping noise. "I'm n-not giving in until this bastard admits defeat...!"
"Likewise, bitch..." Sunny said, briefly removing his tongue from his twin sister's cunt. And with those scathing declarations of intent, they both got to work again
Mari stared at them in stunned disbelief for a moment, then shrugged and began getting undressed
"Well," she began, slipping her blouse off, "I hope you two don't mind if I join you-"
"NO INTERFERING!" both twins suddenly shouted in unison, stopping Mari in her tracks
"Alright, alright, I'll just... do something else, then..."

...

Half an hour later, Mari was lying on Sunny's bed, idly browsing her laptop. She had mostly tuned out the twins' noise by then
Suddenly, something changed. Sunny had started to grunt a little differently, and Mari looked over in curiosity
With a low groan of defeat, Sunny finally succumbed to Haru's ferocious oral onslaught...
Haru yelped in surprise as her brother came into her mouth; he let out so much some of it overflowed and dribbled back down his shaft
When he finally finished, Haru removed her mouth from his cock, swallowed, got up from the bed, and pointed at Sunny
"HA! In your FACE! I WIN! I WIN!" she declared triumphantly (the effect somewhat dampened because she still had some of Sunny's cum running down her chin)
"Yeah, yeah, I get it, laugh it up..." Sunny grumbled
Mari clapped for her little sister. "Congratulations, Haru!"
Haru puffed out her chest and put her hands on hips. "Why thank you, sister! And as for you, brother, I believe we both agreed to a reward the winner of this little competition would receive, but I seem to have forgotten what it was... could you remind me?"
Sunny gave her a look of pure contempt and mumbled something. Haru leaned closer to him and put her hand up to her ear
"I'm sorry, I didn't catch that, would you mind saying it louder?"
Sunny sighed deeply. "Free use for a week..."
"Ah, of course, silly me! How could I forget?"
"Oh, Haru," Mari spoke up, grinning slyly, "not to intervene, but I have an idea you might like..."

A few minutes later, Sunny was wearing a collar. "Property of Harumi Suzuki" was written on it in permanent marker
"You don't seriously expect me to wear this? We're going to Gino's tomorrow..."
Haru just looked at Sunny with a shit-eating grin on her face. "It's been cold out lately, it wouldn't be too weird if you put on a scarf!" Behind them, Mari was barely stifling her giggles behind her hand
"Oh yeah, and before I forget..." She shoved Sunny back onto the bed roughly, and before the boy could comprehend what was happening, Haru had already planted her butt on his face
"Since I didn't lose, I never actually finished, so why don't you get to work on that?"