All characters engaged in sex are over 18.
Sarah climbed off her brother's cock. His rod sprang out of her and slapped his stomach. Sarah scooted down and took it in her mouth, licked it, and said: "This one's for free. Consider it a reward" She looked at Kelly and said: "I'll start him, you finish him OK?"
Kelly was wet and horny. She had been masturbating through the whole episode and had been edging ever closer. She shook her head no -- she wanted him now. Sarah relented, bobbed her head a few times on Rob's cock then held it straight while Kelly sat on it. She looked down at her brother and said: "If you cum before me I'll kill you..." then started rocking as she'd seen her big sister do, and instantly knew why it drove her wild. The clit pressure and the depth of his cock inside her combined to increase her pleasure, she whimpered, rocked some more, and simply came, just seconds after getting his cock inside herself. It wasn't earth-shattering, but it was a release she desperately needed.
Rob was taken by surprise. It had been maybe twenty seconds since his little sister had sat on his cock but she had already shivered her way through a little orgasm. Now she looked down at him, gently riding him smiling her encouragement. He reached for her titties, played with her nipples. Sarah lay beside him, stroking his hair, whispering in his ear: "You dirty fucker, up inside your baby sister like that. Do you like her tight little pussy? Huh? Of course you do. And her lovely titties? See them bouncing? Fuck her hard and watch them jiggle. Yeah, like that..."
She nibbled his ear. She flicked his nipples with her cool fingernails, scratched them along his chest. Kelly was riding him quite fast, willing him to cum. His older sister's dirty-talk helped. "...Now she's got a shaved pussy like mine, see? See how it splits to let your cock in? Yeah. C'mon. Cum little brother. Cum in your little sister..."
With little warning, and unstoppably, his cock suddenly twitched and jetted jizz inside Kelly's welcoming pussy. She giggled at his cum-face, now rocking gently with him deep inside. As quickly as it had started, it ended, and he turned his head to kiss Sarah fully. Then he said: "When did you get this dirty streak?" to which she replied: "Always been there, I guess, but now it's found a way out."
Kelly had her naughty-girl look on her face. Rob saw it, too late. Giggling at herself, she lifted off her brother's cock, sat on his stomach and let his jizz flop out of her onto his stomach in a creamy, soppy mess. Sarah laughed, poor Rob couldn't get up. He couldn't be angry though. His hot big sister and his minx baby sister were everything to him now.
After the first days of exploration and urgent sex, the three siblings settled into a routine of work and play. Each was confident in the other, there was no jealousy. Sometimes all three fucked together, more often it was two of them and the other just gave them their privacy.
They almost got caught once though. Rob had just licked Sarah's pussy when a delivery guy came into the shed. If he'd been thirty seconds earlier, Rob would have been between his sister's thighs lapping up the last of her cum. Sarah told the guy off for not knocking loud enough, then when he left they looked at each other in horror at what might have been.
That night Sarah spoke with them: "Have we got too complacent? Do we give any sign when we're in town or wherever? Shit, I'm so scared we'll be found out."
"Why?" asked Rob.
"
Because,
you idiot, incest is illegal and we're all old enough to go to jail for it!"
She had not been this serious for a long time. Rob knew she was scared and rightly so. He tried to calm her fears: "Well I don't think we do anything outside the farm that would give us away. We don't hug, hold hands, not even a peck on the cheek. So if we keep it here, stay careful, we'll be OK."
Sarah went on: "That's my point. We need to keep it behind locked doors. Rob, no more sneaking up behind me and fucking me in the flower shed. No more pulling Kelly off the tractor for a quick fuck. Unfortunately that also means I won't be able to pull your panties down and bend you over the packing table, Kel." She smiled at Kelly's blush.
Rob spoke again: "I disagree. No wait, hear me out. This is so good because it's free, spontaneous. Wherever it happens, whoever it is."
Kelly agreed with him. Sarah looked at them both and knew they were right. "Oh god, we'll all go to hell."
"Yeah," said Rob, "But they'll let us fuck there!"
Sarah tried to smile, but that night she slept uneasily.
A few days later, on a hot midsummer day, she and Kelly were wrestling with a front tractor tyre. Rob's hand had not properly healed but even so he would not have let them attempt this alone had he known.
They wore jeans and singlets in the heat. Sweat dripped from them, the tyre refused to budge so Sarah called out for Rob.
"Sorry mate. We have to finish this soon and we can't get this bastard off. Is your hand up to it?"
"Yeah, sure. Move over. Kel, hold the lever...there. Sair, grab that. OK,
Pull!