Go get what you came here for: Amy's mouth

You leave after the college girls give you their numbers, telling you that you have a great cock and they're all down to hook up sometime. You gladly take their numbers and tell them you'd set up a foursome this weekend.

Then you leave the parking lot and make your way to the beach, where you see that Rachel and her friend are done playing volleyball and are now tanning and listening to some music by themselves. Which is perfect, because the music distracts them as you creep up on them and watch them as they don't even notice you. *Amy is a total wet dream in her bright yellow bikini, her two brunette pigtails lazily resting on the towel.

Amy doesn't even notice as you get closer and get a nice view of her cute ass. And though she's tanning face-down, you can tell she's got a respectable pair of tits, too.
Rachel is turned over on her side, facing the other way, which makes her surprise all the more unpleasant when suddenly she hears Amy glurking and gacking on your cock. You really impatiently get in there, roughly fucking Amy's cute face so hard her sunglasses fall off onto the ground.

Rachel turns and sees you sloppily facefucking Amy. "Oh David! God fucking dammit dude!" Rachel moans in annoyance. "Of course. How did I not see this coming. Amy, I'm so sorry, David is a dumb, horny idiot who always--"
You pull out of Amy's mouth, and Amy interrupts Rach, giggling, "So this the cute brother you were talking about?"
You look at Rachel, surprised, and she looks back at you in horror. "No I wasn't, Amy! I didn't call him cute, he's fucking gross and constantly horny! Ugh!"
"So you called me cute, huh? Maybe you like my cock more than you can admit," you leer at your sister.
Rachel's blood boils as she snaps back at you, "I didn't fucking call you cute. Amy added that part herself. I called you a gross pig because that's what you are." She crosses her arms in defiance. "And now you're ruining my life by facefucking my best friend, who you PROMISED you wouldn't facefuck."
"Nuh-uh, Rach. I didn't promise you anything. And even if I did, I'll do whatever I want," you say.
Amy giggles with her mouth around your cock. "It's fine Rach, I kinda like him actually." Amy smiles up at you and you smile back.

Rachel, annoyed as you've ever seen her, turns away from you two and she turns up the music on her speaker. However, it does little to suppress the sound of Amy happily getting her young throat destroyed, and her egging you to go harder and faster on when you pull out to give her a break. Though you can't see Rachel's face from this angle, your little sister is clearly pissed that her best friend is on your side already.

*Credit to bryz