Just keep watching...

You decide that that’s a lot to try to explain, so you just stay there, still curious to see what they get up to. After a few minutes, your sister, who you know can’t sit still for five minutes without making mischief, sits up.

“Hey Dave, could you give me a massage?” You sister asks.

“Sure Rach, lie down, the sunscreen will make good massage oil.” Dave responds.

Rachel takes the sunscreen from Monica and hands it to Dave, then lies down on her stomach. Dave gets up, and straddles Rachel’s lower back, sitting on her bony little butt. He sprays a bunch of sunscreen on her back and shoulders and starts rubbing it all over. Then he settles in to massaging her shoulders firmly. He looks comfortable, so it’s probably not the first time that he’s done this, but that doesn’t stop his dick from starting to get hard. Monica tilts her head and glances over.
“Oh boy, I wonder who is enjoying this massage more?” Monica says mockingly, eliciting a slight laugh from the other two.

Dave moves down Rachel’s body slowly, until he is rubbing Rachel’s lower back. Then he slides himself down over her legs, and starts gently kneading her butt with his knuckles. You can just barely make out the look on Rachel’s face, but she is clearly enjoying this. After a minute of that, Rachel starts wriggling her butt and sticking it up in the air, and Dave clearly takes her meaning. At first he just rubs her lips a little bit, and then slightly sticks a finger in her. Apparently satisfied that she’s ready, he lifts her butt up a bit, lines himself up, and then thrusts down on to her waiting hips. He withdrew slowly, and then thrust himself in roughly again.

Rachel’s reaction was immediately, her hands had balled up bunches of her towel.
You shocked yourself with the realization that you were turned on by watching your little sister, who looked just like a tiny version of you, getting fucked.

Nevertheless, there you were. Apparently you weren’t the only one either, as Monica decided to slide over a little, positioning herself in front of your sister. Rachel apparently had been waiting for Monica, and immediately positioned her hands to spread Monica’s legs. Monica was quite the fire-bush, and her pussy was quite pink. Rachel dove right in, tonguing Monica from her opening up to the hood of her clit. Then, with each hard thrust of Dave’s, Rachel’s face was smashed against Monica’s pussy, which neither of them seemed to mind.

Monica, who had nothing else to do, had her hands on her breasts, and were squeezing them, and pressing them together. After a few minutes, Monica couldn’t stop herself from lightly thrusting and grinding against Rachel’s mouth, as she had an orgasm. Once she had finished, Rachel turned back to Dave.

“Okay… okay… let’s finish together.” Rachel said.

Then, Dave’s thrusting, which had been quite methodical, slow out and hard in, became frantic, and Rachel pressed her skinny frame back against him. Monica crawled down to where the two were joined, her breasts swaying underneath her, and at the last moment, Dave pulled out of Rachel, and Monica took him in to her mouth. Her eyes went slightly wide as he came in her mouth, but she continued to move his cock in and out of her mouth, cleaning it. Then Rachel got up on to her knees, moved back to Monica, and kissed her full on the lips, the juices of all three of them on their faces.

You started to swim away, just as the happy threesome collapsed back to the sand. When you got back to the beach, your stuff was just where you’d left it, and the guys had left. Feeling a little bad that you were all alone, you masturbated to the thought of your sister getting sandwiched by a guy and a girl, and fell asleep.