Offline/Try to be more playful
With Emma's arms crossed and supplementing her pillow to help support her head she has really elongated her figure and thinned out her sides beyond her normally scrawny frame. It's probably also why she was so easily tickled before. The wispy structure of her frame is so easily apparent now. She just feels so small and delicate beneath your touch, which you don't mind given your newfound love for everything young and tiny. But since there isn't a lot of meat on her there isn't much worth massaging either. She sure doesn't seem all that into it.
Worried she's getting bored, but still wanting to push this to other places, you pause and think for a moment.
"Hey sis, do you want to try one of those shee-at-soo massages?" You think that's a thing. You're sure you've heard it somewhere, and although you don't actually know what it is, that doesn't really matter.
"Well... okay!" she replies cheerfully.
You shuffle off of your sister's thighs and kneel beside her. You promptly slap your palms against her back over and over inducing a surprised giggle.
"Whu-ugh-yuh-yuh-yuh-yuh-yuh..." she drones, her voice wavering with a broken tone after every strike of your hand. She plays it up, exhaling a flat "Uhhh" with every breath so that you can play her body like an instrument. This is certainly more fun, but you need to mix it up too. Hooking your hands on her far side, you roll your sister over by pulling her toward you then slap at her chest in the same rhythmic manner. She draws her arms down at first to protect herself but then relaxes. She makes no attempt to protect herself which lets you go crazy. She keeps up her steady droning to hear her own voice shake and wobble, between her giggles of course. Asha even laughs a little from the sidelines.
"Okay, full body shee-at-soo!"
Having fun now, you pummel your sis over every little inch of her bare body, up and down her torso, her legs, you even smack her face just because you can. But of course you make a point of patting down her pantied crotch every time you cross the border. She just watches you with a dumb grin the whole time, letting your hands slap and wander across her skin freely.
Rolling your sis face down again your repeat your front performance on her back, slapping her silly. You slap her butt firmer and more purposefully than you did her crotch, and deliberately work in a few squeezes. There's no doubt that she's enjoying all this silliness more than just a shoulder rub. As you squeeze her butt again you think of another idea.
"How about some deep tissue shee-at-soo?"
You lean in and grab both of her butt cheeks, spreading and squeezing, stretching the middle of her panties between your thumbs. Emma props herself up on her elbows and looks back at you over her shoulder with an astonished grin. She laughs and wiggles her butt a little, taunting you, daring you to keep going. So you push it further. You slip your thumbs into the leg holes of her underwear and kneed into the thickest part of each of her cheeks. But you slide your thumbs closer after each squeeze, gravitating to the center, eventually opening up her cheeks as wide as they can go beneath her thin colorful fabric.
"That's not actually deep sheetsoo!" laughs Emma. "...is it?"
"No, not really. For that... we need to go even deeper!"
Brazenly, you yank your sister's panties off her butt and down her legs, slipping them all the way off and tossing them aside. She makes a weak attempt to reach back to grab them but otherwise lets you strip her. She just buries her face in her hands in (mock?) embarrassment, Asha laughing at her friend's misery. You mount your sister again atop her thighs and immediately grip each of her cheeks, spreading her ass fully apart and staring down at her tiny pink starfish. You pry, pull and stretch Emma's pliable little ass, her tight cheeks perfect little handfuls. When you stretch her ass to its limits you can see her pert little asshole open up for you, just enough to peek inside.
This feels like as much as you can probably get away with, and a lot more than you ever could have done with your sister before this trip. Thankfully Emma is still laughing and looking back at you. Your cousin is also reasonably accepting of all this, Asha having gradually shuffled closer to get a better look.