Justin was coming upstairs to go to bed. He’d been up late watching a baseball game downstairs and since it was summer, his parents didn’t really care how late he stayed up at night. Better to be up late at home than out causing trouble, they figure. As he came down the hall towards his bedroom, he sees a shape crouching in the hallway. Curious, he approaches softly until he can see it’s his younger sister Emily peeking in through the door to their parents’ bedroom. The bedroom door is open just a crack and a faint ray of light is peeking out.

Suddenly Justin puts everything together. He bets their parents are making love again and Emily is watching the action. Despite getting up in years, he knew that their parents still had sex at least twice a week. He could even hear them sometimes from his adjacent bedroom.

Justin stood there for a moment deciding what to do. He hadn’t really noticed his sister sexually before, but she has filled out quite a bit over the last few years while Justin was away at college, changing from an annoying brat to a nicely proportioned eighteen year old. Maybe if he plays this right, he can have some fun with her.

Justin creeps up behind Emily, standing so that he is blocking the hallway and the path to her bedroom door. Emily is so engrossed in the scene that she’s watching that she doesn’t notice Justin at all. As Justin gets into position, he can see through the crack that Mom is sitting on top of Dad while he was massaging her breasts.

When Justin is in position, he taps Emily’s shoulder, hoping that she won’t make too much of a racket when startled. As expected, it snaps her out of her reverie and she looks up panicked at her older brother, then at the door. She bolts down the hall in the only direction she has available – into her brother’s room.

Justin follows her, moving slowly and non-threateningly. “What do you think you were doing?” he says quietly with mock seriousness. “You could get caught.”

“I was just up to get a snack and I happened to see…” Emily stammers. “You won’t tell, will you?”

“No, of course not,” he said reassuringly. “Besides, there are better ways to watch.” Justin moved over to the wall that separated his bedroom from his parents and took down one of the pictures hanging from the wall. Points of light appeared on the wall where the frame covered. Justin peeked in and saw his Mom bent down over Dad’s cock, licking and sucking it.

Justin motioned to Emily to come over and she did. She peeked in another peephole and gasped at the sight. “Wow…” she muttered. This was a much better view than from the doorway. “It’s so large,” she murmured.

They continued to watch as Mom sucked on Dad’s cock, with her full breasts hanging down beneath her, swaying in time to her motions. Dad’s face was contorted in pleasure and his hands were grabbing at the sheets around them.

Dad then sat up slightly and rolled Mom over on her back. He moved down the bed and peeled off her lacy panties and then buried his face deep in her crotch. A soft moaning sound could be heard coming from their bedroom as Dad licked her pussy.

Justin glanced over at his sister. She could see that this erotic scene was having an effect on her. She was squirming in her nightshirt and her breathing sounded a little uneven. Her entire focus was on the sights in that room and she was lost to what was going on around her.

That was probably just as well since the sights gave him quite a boner and it making a large tent in his PJ’s. Emily would probably freak out if she saw it right now. He wondered how far he could go before she came to her senses.

Justin moved behind Emily and rested his hands on her hips. He didn’t notice any reaction from her. She was still lost in her world. He moved his hands up and down her side slowly. On about the third pass, he slipped them under the hem of her shirt and was running them up and down her bare torso.

Still sensing no objection from Emily, he moved his hands slowly to cup her breasts. They weren’t quite as large as Mom’s, but they were more round and firm. They felt very soft under his hands, with small nipples on the tip that were rock-hard.

Emily noticed this new contact and it just added fuel to the emotional fire that was burning through her. She leaned back slightly against Justin’s body, never taking her eyes off of the action in the other room, and allowed his hands to roam all over her full tits.

Sensing this capitulation, Justin sent one of his hands down lower. He found that her cotton panties were already soaked with her juices and when his finger touched the outside of her pussy, Emily gasped in pleasure.

They could both see that Dad was moving up the bed, straddling Mom. His cock was hanging heavily below him as he moved it into position. Dad ran the tip up and down the opening a few times and then pushed it slowly in. Both parents gasped at the penetration as his cock slid all of the way inside on the first thrust.

Justin moved his hand inside the panties and started running his fingers up and down Emily’s opening as well. At this point, Emily’s whole body was squirming back and forth from the sensations, while her eyes remained locked to the other room. She had a great view of Dad’s large cock moving in and out of her Mom’s pussy. The sight of that combined with the feeling of Justin’s hands running up and down her own pussy.

“Would you like to know how that feels?” Justin asked. Getting no response, he slid down her panties with his hand and then pulled his cock out through the opening in the front of his PJs. He pushed his cock up against her opening from behind, stroking it up and down to get her wetness on him. When he was in position, he crouched down slightly and then lifted up, impaling her on his manhood.

Emily gasped at the fullness as he entered her. Any pain from the intrusion was lost in the feelings that were coursing through her. She could feel his cock push deep inside her, and then slip back out again, only to thrust up and refill her.

Justin relished the sensation of how snug her sister’s pussy was. Her body seemed softer and more welcoming than the few other girls from college he’d been with. It seemed to be just the right size for him.

Justin’s pace unconsciously matched the pace of her parents in the other room. As he thrust in and out, his fingers found her clit and started rubbing it, while his other hand held her steady across her tits. He knew that he wasn’t going to last long in this position, and from the moans that Emily and her parents were making, everyone else was close to their own release.

Mom was the first – she thrust her head back against the pillows and arched her back towards Dad’s thrusting cock. Her hands were grabbing at Dad’s back, pulling him tighter to her as her pelvis quivered against his. Dad then let out a moan and pushed hard against Mom. They could only imagine the jets of sperm that were now filling Mom’s pussy.

This sight was enough for Emily as she started gasping in her own orgasm. He could feel her pussy clutch his cock as she collapsed forward against the wall. This was all of the encouragement that Justin needed to make one last thrust deep into her and pulse after pulse of his semen filled her quivering pussy.

After a moment, they all caught their breath. They looked through the peephole again, relieved that their parents didn’t hear any of the noises they made. Emily could feel the cum dripping out of her pussy as she looked back at Justin wondering if what they had just done was a good idea. At least she was on the pill (to make her periods more regular) so she didn’t have to worry about that.

Justin also looked at Emily wondering what she would think now that the moment has passed. “Want to watch again some other night?” he ventured.

Emily thought a moment and then smiled back at him. “As long as nobody is watching us.”

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