Yes/Chad/Place your hand on Emily's thigh
You lean across your shared armrest and place your hand on your sister's thigh. Specifically, you pat the few bare inches above her knees that are exposed. After resting your palm there for a moment you then rub further up her leg, doing what little you can to calm her nerves.
"Hey, relax," you whisper in her ear. "You picked the back row on purpose, didn't you?"
Em looks down at your hand. It's much harder to see in the theater now that the lights are out. Most of her upper body is lit from the screen, but her lap is quite dark. It's still clear where your hand is, and only more apparent as you have pushed the bottom of her dress higher while rubbing her leg to expose more of her flawless skin. The closest people to you are several rows from the back and a few seats over, but even if they were in your very row they'd be hard pressed to notice where your hand is if they weren't seated right beside you. And yet she's still nervous.
"Umm... maybe," she admits, biting her lip and glancing around before looking down at your hand again. "Maybe cuz it's... private."
"Totally. We have lots of privacy." You firmly grip the inside of her thigh. Em postures up, licks her lips, and takes in a big breath, her focus fixated on your hand. You bring her attention back to you. "Hey. It's just like you said. We can do anything we want back here, can't we?"
Emily starts to smirk, then bites her lip and glances around once more before hooking her arm around yours and nuzzling up against your shoulder. You pull on her leg, hoping to make more room for you to go higher, but Em just scooches closer to you before clamping her other thigh around the back of your hand.
You're disappointed, but you don't mind if she doesn't want to go further than this right now. If you wanted to force her into something you would just slip the ring on. So you reach over with your over hand and rub her forearm sweetly, while the thumb on your clamped hand occasionally strokes the top of her thigh.
The movie is okay. It follows two groups of friends trying to define themselves in their last year of high school. One guy wants to become a legend, someone that new high school kids will talk about years after he's gone. A girl wants to lose her virginity after discovering she's the only one of her friends who hasn't. It's all pretty typical, but the characters are at least likeable.
There's a pool scene 20 minutes in. The girls and guys mingling for the first time at someone's birthday party. The girls are wearing somewhat skimpy swimsuits for a birthday party, and the most ripped guy is in a literal speedo. There's some jokes about pubic hair and peeing in the pool, but those are the least interesting things that happen as this scene gets underway.
Emily takes a moment to adjust herself, pulling away from you briefly while she tugs on her dress. When she grabs your arm again she slinks lower into her seat. At the same time she pointedly pulls on your wrist, guiding your hand further up her leg. It's not as if there's anywhere else for your hand to go; you immediately bump up against her panties.
You tilt your head to the side to look at Em. She glances at you after a moment, but so briefly to suggest she only wanted to know if you had noticed something. Peering down into your sister's lap you can tell her dress is hiked up quite far. She may no longer even be sitting on it. That means every inch of her legs are now laid bare.
Following her lead, you take a moment to shift as well, rotating your hand in the process to face into her pantied crotch. When your hand presses against the inside edge of her opposite thigh she parts her knees for you, kicking her far leg out to one side and leaning into you a little harder, bracing herself against you as your fingers come to rest against her mound.
One thing is clear: your sister is quite horny. The crotch of her panties are very wet, like they're trying to hold back the slow drip of a faucet, the plain cotton underwear soaked through in a prominent oval pattern that you trace out with your fingertips. Em squirms when your fingers first tease her then lets out a big sigh, as if she can finally relax now that you're touching her.
You both stare straight ahead while you paw at your sibling's radiating mound. On screen, the virgin girl follows speedo into the house and tries to corner him, wanting to flirt, but she quickly loses him inside of what is a mansion anywhere but Hollywood. Meanwhile the legend-seeker jumps off the roof into the pool but loses his trunks on entry. He scrambles out naked and takes off toward the house seeking cover, but slams into the virgin as she comes back out, the two of them landing on top of each other on the patio in front of the party.
"Woo! Go Julie! Lose that v-card!" one of her friends shouts.
Em tightens her grip on your arm and pushes her pelvis into your hand before resting her head on your shoulder. You respond by pressing against her mound more firmly, working a steady tight circle over her slit.
The movie goes on and you diligently please your sister. You press in on different spots, trying to gauge her reaction. You dance along the hem of her panties, skirting over the crook of her dry inner thigh before pressing into her wetness again. Em intermittently pulls her knees together - perhaps overwhelmed by your continued focus, or just needing to squeeze your hand. She'll twitch or writhe or silently grunt then open her legs back up for you to continue.
This is the most risky and exciting thing you've ever done in public, and without the ring (sort of, not really). You're crazy hard and she's melting into her seat. Emily lets out heavy breath after heavy breath until a chase scene comes on, something about saving a mascot. Screeching tires, loud music and frantic yelling fills the theater. Your sister takes the opportunity to sit back and softly moan, just a little, her head all the way back against the headrest, her eyes closed, her mouth open, as if drunk off of your sustained fondling.
You could probably do anything just about now