Hot Summer Job/Antagonize your sister more directly
You let go of your sister's privates to reposition further. Grabbing her wrists again, you lift her arms up and tuck her hands in behind her head. This might help her relax, but it definitely stretches out her torso once more, her already flat tummy sucking in a bit to show off her hip bones and accentuate her pubic mound. Then you grab a knee, pulling it out to the side and manipulating her leg into an L shape, really opening up her pelvis. Her other leg you push open as well but barely need to move it to gain unimpeded access to every inch of her sex.
"If this vagina-nipple theory of yours is correct," you start, snaking a finger across the words you've written, "then we need to test them both separately and together."
"Can do, Johnson," you reply. Without another word your grab at your sister's chest with both hands. She squeaks, surprised by the suddenness. Bree's undeveloped chest has a certain malleable quality despite being so flat. You palm her tits for a moment, enjoying the feel of her tiny nipples poking into your hands.
"Let's pay close attention for a response Johnson."
Moving your thumbs into position, you work her light pink nubs like joysticks, spinning around and around. You keep an even steady pace, pausing only so you can flick them back and forth on occasion. At one point you lean down and give one nip a firm lick while continuing to circle the other. Bree remains perfectly still and rosy cheeked through all of this, the only real observable effect on her breathing patterns, getting deeper and stronger. What's obvious is how hard she's trying to look expressionless.
"Hmm, I can't see anything. Let's move to the next stage."
Quickly securing a hand over Bree's pussy, you part her young lips and threaten her entrance once again with a fingertip. You notice a quick gasp when you first poke at her, but you don't call attention to it. You instead tease your sister's tight hole. It takes no time before you can feel her pent up excitement slowly trickle out of her. There's no doubt she's managed to get herself worked up by this game and is impressively slick. Although tempting, you don't enter her. Instead you pull her wetness up onto her concealed clit.
Pressing a thumb atop her mound allows you to pull back on her hood and expose her tiny sex button. Slathering it in her own juices, you firmly press into and spin round her clit much like her nipples a minute ago. Her leg bobs faintly, just for a moment, before she gets her latest outburst under control. Running with that bit of encouragement, you speed up and intensify your focus on her extra sensitive little spot.
Bree's mouth is opening wider and wider as time passes. You can see her eyelids flutter every few seconds, her breathing getting slightly faster. She's certainly starting to enjoy this, as if she wasn't already. You sure are. Your cock has been at full mast for quite some time, held back only by your shorts.
This truly is insane now. Here you are, openly masturbating your 10 year old little sister, and she's still stubbornly playing dead, refusing to acknowledge the sinful, reprehensible acts you're performing on her. What do you have to do to wake her up?