Offline/Tell her to kiss your dick instead
"Actually Bobby, if you're looking for something to kiss... you can kiss my penis."
"Ew!" Beth recoils, her nose wrinkling. Then, intrigued, she ponders the offer. "...Why would I want to do that?"
"Because it feels good," you plainly admit, before adding, "And I thought you might want to. You're the one who wanted in on a circle jerk."
She ponders this some more, a silly grin creeping back onto her face while you all keep stroking your respective equipment, before agreeing to try it.
Beth shuffles past Brayden then leans down toward the tip of your dick. Her baseball cap obscures your view as she gets her face within inches of you. You stop jerking, pulling your hand away to give her room, but really that gives her pause. She stops and sits up, making a funny face. She glances between you and her brother, perhaps wondering if she's being tricked, then goes through with it anyway. Only this time she tilts her head as she goes down and gently kisses the side of your shaft. She does it the same way you instructed her though, mouth slightly agape, then pushing forward with her open lips before slowly closing them, successfully completing her latest attempt at a sensuous kiss against the side of your cock, a fact that sets your brain on fire. She sits up trying to hold back her nerves and her smile, again glancing between you and her brother as you all resume masturbating.
"That was... I dunno," she shares, not sure what to think of the latest twist in this situation.
"Oh, um... that's 'cuz you did it wrong," you inform her.
"Whaddya mean?" Beth asks. Her brother looks equally curious as to how she screwed anything up.
"You're supposed to kiss the end of it, not the side. Right on the top. You gotta commit to this."
"But that's even weirder!" she giggles, bringing her hands under her chin. You don't reply, letting her stew over that thought as she stares down at your dick. "Okay! Okay! I'll do it!" she blurts out, as if you were all just egging her on.
Beth bends down over your lap again, straight on with your cock. You hold it steady for her this time, for when she pushed against the side of your dick before, it moved away from her mouth limiting how much pressure she could apply. That's less of a concern coming straight down on it, but you still want her to hit her mark.
Her mouth contacts the slick tip of your cock, your precum more than plentiful with how long you've been stroking, pushed up into a little pool by the lip of your foreskin while you hold it steady. But she isn't phased this time and follows your kissing instructions to the letter. Even though her cap is still in the way, shrouding her actions as it bumps against your stomach, you can feel every single thing that she does.
Beth relaxes her jaw before pushing against your tip, your glans slipping imperceptibly (to her) just inside her mouth as she finds the edge of your foreskin with her parted orifice. Her thin lips press in a ring around your glans, unintentionally pushing your foreskin halfway down your head as she tries to apply pressure and your tip peeks inside her mouth. The tip of her tongue dips into your precum and connects with your urethra, with it beginning to push at the rounded end of your glans like she's trying to keep you from entering any further. That tiny bit of panic from her tongue is all that keeps her from fully taking you in like a cock-sicle as she puckers up, her lips parting wider, no doubt tasting you fully as she still goes through all the detailed motions of how to properly give a sexy kiss. Your sheath follows her face back up to the end of your wet head as she slowly closes her lips, an action that feels like she's pushing your glans out of her mouth, since that is what she's effectively doing as her closing lips slip off of the end.
Despite having just made out with your foreskin, she goes through all the motions again twice more of her own volition, albeit a little faster. She may be afraid of not doing it well enough, or she may have gotten the courage to redo it how she intended the first time, but this concerted effort sure is nothing like the shy peck she gave the side of your shaft a moment ago.
Beth pulls back quickly once she's done but she takes a prize with her. A long thick string of precum stretches from your dick to her mouth as she pulls away, her lips thoroughly coated with it. The strand snaps shortly after her face comes into view, the bottom half of the severed rope of clear fluid wrapping over the back of your knuckles with the rest wrapping underneath Beth's chin. She merely giggles at the "drool" as she wipes at her mouth.
"Did that feel good?" asks Brayden. He's asking you, but Beth is the first to answer.
"It, um... made me feel funny!" admits Beth bashfully, gripping her water bottle tightly between her legs.
You slowly pump yourself a few times, sliding your foreskin along the path Beth's lips took. "Yeah, that's... more like how you do it."
Brayden frowns as he glances between the two of you, jealous of his sister once more. "Well I wanna try, too. I can do whatever he does."
"Seriously?" You're surprised he would be willing to go so far to one up his sister, but far be it from you to turn down a warm mouth on the end of your cock. "Be my guest," you chuckle, angling yourself toward Brayden.
Brayden tips himself straight over, diving down towards your dick. But, despite his speed, he's more timid than Beth and is softer and slower than she was, his nerves stopping him once his lips meet the opened ring of your foreskin. All his eagerness is quickly tempered by taboo and his own self awareness, things his younger sister seems slightly less encumbered by. That means it takes him twice as long for him to get half as far as his sister, still nursing his second kiss in the same amount of time, but in some ways he does a better job. His tongue doesn't fight you like Beth's did. It reaches out to meet you and then reaches under you, skirting with your frenulum on the second try, his kiss enveloping your slick tip all the way to the edge of your purple helmet. That's when his younger sister butts in and kills the moment.
"Boys are so weird," she says while diligently stroking the penis bottle between her legs. Her brother hastily rights himself after that comment, as if he wasn't just sensually blowing the very tip of your cock, then hurriedly licks and wipes his lips. All you can do is grin at the pair of cocksucking cousins you just managed to christen while on family vacation. It's curious to think that pitting them against each other might even work in your favor. But this is already way better - and weirder - than you ever could have imagined this week could be.