Offline/Silently mock Jet as he talks
If she's already amused by you and you haven't even done anything, why not ham it up? You start mimicking your cousin, nodding your head with him, mouthing his words, exaggerating his gestures. You're just silly enough that it starts to work. Soon his girlfriend can't hold back her smile any longer and begins snickering beside her boyfriend. Jet does eventually catch on, turning to face you across the aisle.
"Hilarious," Jet replies dryly.
"Well your girlfriend thinks I'm funny." She quickly sucks her lips into her mouth, still struggling with her smile.
"Apparently," he notes. He doesn't sound mad, more annoyed that he's surrounded by such immaturity. He is older than you, so he's at least pretending to be above this sort of thing. "Hannah, this is Justice. One of my many weird little cousins."
"Call me Jay," you reply, extending your hand past Jet to shake hers.
"Oh, you're going by Jay now?"
"Nice to meet you Jay! Your family really loves their nicknames."
Jet shakes his head. "Nah, our parents just keep picking boring names that either no one wants to have or no one cares to call us."
"Oh! Do you think I'll get one too?"
"After a week? Probably," you nod.
"I don't know if she's weird enough to qualify."
"C'mon, I don't think your family is that weird! They're actually really nice! So far. And you haven't even met mine, so you have no idea! There was this one time when my sister was younger that she went a whole week pretending she was a turtle! She tied a couch cushion to her back and crawled around the house doing everything in slow motion!"
"Now that's actually hilarious. A whole week?"
"Yeah! Longer even! Ohmygod... momgotsomad! She couldn't. Even." Hannah then bursts out laughing before rambling on about her turtle sister for several minutes, being sure to keep you in the conversation. She meanders from there, going all over the place with her stories. You didn't come back here to simply chat with them, but Hannah is more than happy to include you if it means you're listening to her. Eventually you do manage to work in your offer naturally.
"...but that weekend was pretty boring. I've never really even been camping before. Not like this," Hannah points out the window into the dense forest, branches occasionally brushing the side of the bus. That's your opening. You're quick to jump in before she changes topics again.
"I have! I've actually been to this camp before! It's not a 'real' camp anymore, it just used to be. But I helped find it with my dad and Uncle Dave. It's pretty good, but I'll let you in on a secret." You lean even closer from across the aisle, really just squatting in the aisle now, for dramatic effect. "There's a small cabin hardly no one knows about." Not really true. "It's way outside of camp." Very true. "My own personal cabin." Very not true. "But... if you wanted to share my cabin with me for the week I'd be down for that. You're both pretty cool." You watch their amiable expressions fluctuate a little as you fail to wow them. But you're quick to add, "You'd have a lot more privacy than you would in the main camp sharing the larger cabins."
The two of them stop and look at each other in unison, psychically communicating their options. A moment later Jet replies.
"Alright. Sure cuz! Thanks for the offer. There's room for all three of us out there?"
You sure hope so. "Yeah! Definitely!"
"Good stuff. Especially since we're already here."
You turn toward the front of the bus just as the trees break, wood cabins coming into view. You freak out a little, mostly with joy. You tell Jet and Hannah to follow you when you get off the bus then race back to your parents to tell them that your older, mature, responsible cousin will be staying in the cabin with you. They both seem fine with that which is as much as you can hope for. Doing a mental fist pump of victory you prepare yourself for an awesome week.