Offline/Make him kiss his sister first
"Uh, you should... kiss Beth first," you say with a hint of fear. You're not sure what you're scared of exactly. Maybe liking it, even though you're sure you wouldn't. "She, um... Beth's the one razzing you about not kissing a girl."
"I'm actually Bobby right now," insists Beth.
"Yeah, see? Exactly," you continue. "So you'd still be kissing a boy, too. It's like kissing both at once."
Slightly dejected, but easily persuaded, Brayden turns with a frown to his sibling and shuffles within range of her. She's just grinning again. Their kiss is far more awkward, Beth giggling a little and Brayden trying to make her take it seriously. They break after a short 2 seconds, but Brayden already has strong opinions.
"Yeah, now I know you're lying because you're a bad kisser."
"Am not! Why am I a bad kisser? Jay, am I a bad kisser?"
How are you once again in the middle of these two? Can you not even jerk off without them fighting over you? You let go of your stiffy and sit up a bit. "You... look, you're not a great kisser bu-"
"Nuh-uh!" she says, already moving toward you again. "I can prove I'm a good kisser!"
There's no talking her out of it. You find that the second time she kisses you she's even stiffer than the first. All she does differently is press against you harder which does not make for a more pleasurable experience. You're far from an expert, but you're positive you have a better idea of what makes a good kiss than she does, and she really needs to know it.
"That, um... yeah, that was worse than before."
"Told you you're bad," huffs Brayden, bitterly bating alone now.
"What do you mean? What did I do wrong?"
"I don't know exactly. That was like a kiss you'd give Gramsy... if someone pushed you into her. I think you're just trying too hard. It should be softer, but... excited. Think, like, sexier. You're supposed to open your mouth a little and then slowly close your lips, but not your teeth. And then, well... you do it again. I dunno if I'm explaining it well." Doubly true since you have about as much real world experience as these two. "Basically it's like a weird dance with your mouth."
"Uh-huh... I can do that," she assures you after a short think.
She's ready to try immediately but thankfully her third attempt is much improved. She's a bit slower, in less of a rush, lingering with her initial smooch before initiating another one. It's so much better in fact that you resume masturbating mid-kiss, her dainty little lips finally sharing the unschooled passion she has within herself.
"How's that?" she asks, looking bashful.
"Yeah, that was better. Lots better."
"Hear that? I'm a LOT better than you!" Of course it's still a contest. Everything is with these two.
"No you're not. He's being nice... show me what you did with him."
The siblings square off again, getting closer this time, so close that Brayden's boner bumps Beth's bottle. He's supposed to be testing her but looks more nervous than she does and, from where you're sitting, she does a much better job facing off against her brother the second time.
"Now you're the bad kisser," teases Beth, even more confident in herself.
"No I'm not! Jay has to decide... but he doesn't want to do that, so... you're just wrong, okay!"
"C'mon Jay!" pushes Beth, grabbing her brother's arm. "It's the only way to pick who's better!"
Rolling your eyes and waving him over, Brayden perks up but looks as nervous as ever. You're still reluctant, but if you're only playing judge then that's not so bad, right? That's not gay, right? Except when he bangs his knee against yours and goes all wobbly and you reach out to grab his side to steady him except you happen to keep your hand on his waist when he grips your shoulders and looks deep into your eyes before he closes in for his kiss.
And it's... okay. If you're judging. Kissing him wasn't so scary once you got it over with. You even got a little tingle up your back, but he's clearly the most nervous one here. Maybe his attempt was better than Beth's first kiss, but he's not better than her most recent one, though you decide to still masturbate all the way through Brayden's attempt anyway. A kiss is a kiss and you've never been able to make out and jerk off at the same time before. Yesterday your balls would have been downright aching if you were just kissing someone while naked, and here you are actively masturbating with these two at the same time. Besides, at the very least you want to come across as an impartial judge. It'd be impolite not to jerk off while he kissed you.
Brayden pulls away from you pensively, his dick straining and flexing in the air all on its own as he waits for your verdict.
"I dunno. You're both pretty good actually." That's not especially impartial, but it is kind.
Brayden gives you a shy grin before cockily shaking his head at his sister.
"He's just being nice. I'm still a better boy than you."
"You're not a boy."
"And I'm still better than you. C'mon Jay, ol' Bobby will really prove it this time!" She moves to kiss you again, giddily playing with her bottle and thoroughly enjoying this game now. So much for shocking her.