Warning: this story is a fantasy and details sexual behaviour that is, at best ill-advised. But it is pretend, so so long as you aren't a doofus, you should be ok. All sex-acts described are within ages of consent in the country it's written in (England, for the record).
I'll try to keep the parts and the action coming!
*
"I'm going out -- try not to behave like a toddler whilst I'm gone!" Penny winced at Hannah's sarcasm, but she could understand her older sister's mocking -- no 23-year old should be wetting the bed. Of course, what Hannah, 25, did not know, was that Penny hadn't wet the bed -- it had been Natalie, their 19-year old step-sister. But Penny couldn't tell or Natalie would reveal that Penny had groped her boobs whilst masturbating during the night. Of course, Penny could then reveal that Natalie had since licked her pussy whilst fingering her arsehole, but she hardly wanted to discuss that with her older sister!
Penny was still just wearing her robe, her sleek, dark hair tied loosely back. She walked through to the kitchen where Nat was making a coffee. She smiled as Penny walked in. "Hey, you want a drink?" she asked. Penny shook her head.
"Look, Natalie, what happened upstairs..." Penny's words were silenced by the soft lips of her younger sister as she embraced her, kissing her full on the mouth whilst slipping her hands down her back to cup her bottom. Penny knew she should pull away but her body again betrayed her, and she pressed against Natalie, returning her kiss with a passion. The dark-haired girl gasped as Natalie broke the kiss, only to start to kiss her way down Penny's slender neck, pulling her robe open to expose her pert, C-cup boobs. As Natalie's lips and tongue trailed across Penny's nipples, the older girl suddenly seemed to realise that they were standing in the middle of the kitchen.
Taking a quick step back, she clutched her robe closed. "No, Nat, this is wrong! This is bad!" she said, urgently.
"Bad?" Natalie pouted. "Felt good to me." She stepped forward, intending to renew the embrace, but Penny pushed her away.
"No! For fuck's sake, you're my sister -- we can't do this."
"Why not? You liked it -- I know you did, or you'd have told Hannah that it was me that wet the bed. You liked being bad with me." Natalie's expression was salacious, and Penny could feel a totally undeniable heat between her legs.
"No, Nat, no." This time there was a lot less conviction in Penny's voice.
"Well, if we were bad, then it was mostly me," stated Natalie with a smirk. "Sure, you started it, but I wet the bed and then made you take the blame, plus I licked your pussy. So it is me that's been bad. Maybe...maybe you should punish me?"
"Pun...what? Nat, what're you on about?"
"Come on, Penny -- you said we were bad, but I was the really naughty one. All you did was touch your sister's boobs, but I was much worse. So punish me."
"But...I...I mean, how? What are you talking about?"
Natalie grinned. "You know -- don't you think I should be spanked?"
"Spanked?! No! Where do you get that from -- we were never spanked, Natalie!"
Natalie gave her older sister an arch look. "Well, you can hardly stop my allowance or ground me. And if you send me to the naughty step I'll wet myself!" Penny gasped. Seizing the moment, Natalie took Penny's hand and pulled her through to the lounge. The front window looked out on a private garden so, unlike the kitchen, they were not overlooked. Natalie guided Penny to a stunned sitting position on the edge of the couch, then turned her back on her. Slowly, the voluptuous teenager raised her short skirt to present her step-sister with a view of her peachy bottom and her stretched, pink gingham cotton briefs. "There, now, don't you want to spank it?"
Penny wanted to say no, but was overwhelmed by her sexual desire for her sister and was swept up by the younger girl's seemingly very kinky side. "Do...do you want me to?" she stammered. Natalie took this as a sign of progress and, before Penny could react, lay across her sister's lap, presenting her round bottom cheeks to her.