Offline/Try to get Beth talking to keep her awake
Keeping her awake is the only chance you have at doing anything. So as soon as your head pushes into your pillow, before she can get sleepy, you try to chat with Beth. Not about anything in particular, just all sorts of things, digging into her life, asking plenty of questions and trying to find something she wants to talk about, and then letting her talk at length. You'd love a better idea of where her head is at as you just can't believe that she isn't even a little interested in fooling around with you.
She might be tired, but she's plenty happy to talk once you get her going. You don't mind listening either. You'd do a lot more than that to get your dick sucked again. But you find it's almost as fun to idly play with your cock in the dark while laying next to Beth. It's a weird sort of power trip to be laying next to a girl, gripping your cock, and have her going on about some vacation or something, completely unaware of what you're doing. It's even more of a treat to be lying next to her totally naked.
You swear life as a teenager would be so much easier if you could just whip out every boner the moment you got one and beat it into submission rather than having to hide them all the time. The freedom of being able to beat off in math class or in the hall between classes. You're not flat out jerking it now though, just playing with yourself. It's not difficult to keep yourself hard when you're laying bare ass nude next to a girl who is only in her panties, both of your sleeping bags wide open. Even just hearing her cute little voice get excited as she talks seems to make your shaft flex all on its own.
She brings up her brother a lot, apparently using him as the benchmark to help prove how cool all of her experiences and interests are and how much better she is at this or that. They compete way too much as far as you're concerned, and they'd likely be better off if they had separate rooms at home. And yet, at the same time, she clearly loves him, looks up to him, and is proud that he's her sibling. You have to admit that 90% of the time they do get along and play together great, but that remaining 10% can get real annoying. Antagonistic dick-sucking is a big plus though.
Beth also brings up how Auntie Grace stayed with them last year and taught her how to cross stitch. You can't help but use that as a foot in the door to share how you stumbled across Auntie at the beach earlier, but it doesn't faze Beth at all.
"Yeah! Auntie loves swimming!"
Huh? Was she not listening? "No, I mean she was getting naked at the beach when I went down there. Like, all the way."
"But she didn't?"
"Not after I showed up. But she was still partway naked. She only pulled her bottoms back on."
"How come?"
"How come what?"
"How come she covered up?"
"Well... she was embarrassed I caught her, I guess."
"Huh. Okay. But if she was, then why not cover up all the way?"
"Uh, honestly? I think she kinda liked it."
"Yeah, that's why you're not making sense."
"What do you mean?"
"Well she's always naked at our house."
Is that so? That's... something worth digging into, but you don't want to get distracted either.
"Er... that's not... I mean she liked it this time. Like she liked having me look at her like that. Just like how you and Brayden liked kissing me so much."
"And penis kissing you!" she teases.
"Yeah, exactly. And there's ways to tell when a girl likes something."
You let this hang in the air for a moment. Beth doesn't take long to bite. "What's that?"
"Oh, there are ways. Like sometimes, when girls get excited about something, their nipples get hard."
"No they don't!" she insists, slapping your arm, confidently speaking on behalf of all members of her sex.
"Do too. And when they get hard they stick out more. And hers were way out. Super pointy."
Beth rolls her eyes at you. "Quit making things up."
"I'm not. Okay, here." You reach across and place your hand atop her torso. Although you can see her in the moonlight, you can't see much given the angle the light is coming in through the window. So she lets you drag your fingers across her upper body until you're sure you have her nipple, a diminutive thing that barely registers as feeling any different than the rest of her undeveloped chest. You pinch and rub it a little to make extra sure, making her giggle, then take some fake measurements with your hand. A moment later you hold two fingers up in front of her face.
"Hers were stuck out that far."
"No way!" she exclaims, elbowing the arm holding your dick. "You're just a fibber!"
"Am not. I can even prove it to you."
"Sure! Prove it!"
"Alright. But I have to kiss you."
That surprises her. You can just see a giddy grin work its way up her cheek. "Huh? You... have to.... Um, okay!"
Pulling up slightly, you prop yourself on one arm then lean over your young cousin. You can tell she's wearing a huge smile as you bring your face up to hers. When you lock lips with her your fingers feel for her nipple again. You rest your digits there as you kiss, Beth stumbling a moment before she remembers all of the motions of the "sexy" kiss that you taught her she's supposed to do. But you don't go for too long before you pause.
"Hang on a sec," you say after breaking your kiss. Repositioning yourself, you bring your lips down onto her nipple. She giggles softly as you suckle the tiny patch of flesh, then giggles loudly when you tongue her nip more forcefully. After getting her nipple good and wet, you slide back up to resume your kiss. Your digits start rubbing a small circle over her flat tit while you swap spit, gradually coaxing the very center of the tiny nip to stiffen. And she knows it.
Embarrassed, Beth starts to turn her head away from you and giggle some more, but she lingers long enough for you to get in one last kiss, your ballsack tightening after your first proper make out session with a girl. You finish up by blowing air across her chest, hoping to stiffen her wet stimulated nipple even further by cooling it. For a point of comparison you slide your hand across to find the second little nub, but it's just as stiff as the wet one.
"See?" You bring Beth's attention to her slightly more prominent stiffened nipple by rubbing and gently flicking the inexplicably firmer flesh. It's a tiny difference, but you've proved your point. She just keeps giggling and slaps her hand down over her boob, pushing your hand away. "That's proof of how much you like kissing me. I think Auntie Grace likes being naked even more than that."
Beth considers this idea in silence, having not only just learned something new about her body, but having something exposed that she may have thought was private. Wherever her thoughts wander, they end up in a very specific place a moment later.
"Can I look at your thingy again?"
"If you want to, yeah. It's not like I'm hiding it."
Beth grabs the flashlight and points it down at your junk. You push your erection over to one side, pretending to angle it for better viewing while making it less obvious that you're solidly erect.
"How come it's so much bigger than Brayden's?"
That's a nice stroke to the ego, but you want to be careful how you respond to these sorts of questions, in part because you don't know where she's going to go with it.
"Well... I guess because I'm older, but I don't know if I'm that much bigger."
"You're not all weird like my brother is, are you?"
What's that supposed to mean? Is she calling you out for playing with yourself? "Umm... I think all guys are a little weird when it comes to this sorta stuff."
"But do you do any of that weird stuff he does at night?" Does she still not get what he does in their room? Even after watching him jerk off this afternoon? It almost sounds like she's asking you because she's hoping you might be able to give her some answers.
"Yeah, well, uh... Only when my girlfriend stays over." Like you've ever had a girlfriend.
"Oh! So he's pretending he has a girlfriend?"
"Maybe... I guess that's one way you could pu-"
"Can I pretend to be YOUR girlfriend?"
"You would? Really? I mean... Yeah I guess so. If you want."
"I'll be the BEST girlfriend! You'll see! But you have to show me what you do when your girlfriend stays over. I bet there's lots of kissing!" Her tone suggests she's teasing you for wanting to. Did she already forget what you two were doing a second ago? That's the level of maturity you're dealing with here.
"Yeah, there's a lot of that stuff... and we, y'know... sleep together."
"Okay!"
"Yeah?"
"Uh-huh! I like cuddling at sleepovers."
"Oh..." Yeah, she's still pretty innocent, even after everything you did with her today. You're starting to feel bad that she might be this naive, but every time you look down at her bare shoulders, girlie little panties, and cute teeny toes, your dick immediately refocuses your thoughts. Beth sets the flashlight down then wiggles her butt off of her sleeping bag to shimmy herself right beside you atop your splayed open bunk, her leg bumping the tip of your tilted cock as she literally rubs shoulders with you.
"I bet you and your girlfriend do everything together, don't you?"
"Oh yeah! And then some."
Beth wears a broad smile as you stare at each other, but it's suddenly wiped off her face. "Whatcha mean?"
"Huh?"
"What's then some?"
"Oh, I dunno. We just do lots of things."
"Like what?"
If you'd ever had a girlfriend this would be much easier to answer. "Uh... stuff."
"Like what!"
Ah geez. She's shaking your arm. How are you going to direct your innocent little cousin?