Confront Her Directly

You keep the video open on your laptop and walk it upstairs. You knock on Elise's door.
"Elise, it's me. Can I come in? We need to have a talk about something?"
"Sure." She replied, opening the door and gesturing you to take a seat on the bed. "Consider me intrigued, what do we need to talk about?" She closes the door and you take a breath, not sure how to begin.
"Well, I found something that I'm not sure you'd want your mum to see." You say, opening the laptop and playing the video. The look on Elise's face changes from curiosity to shock to embarrassment all within about a second.
"Okay, enough, put that away. I don't need to see it. I was there." She snaps, in a soft, panicked voice. "Why are you showing me this?"
"I want you to know that I know about it." You reply. "You love winding me up, and teasing me, and eating my food and taking my stuff, and making me drive you everywhere. Well, all that stops now, or I'll show this video to Cassandra."
"Fine." Elise grumbled.
"You know what?" You continue "If you do anything to annoy me again, I won't just show this video to your mum, I'll show it to your principal. You might be 18, but you are still in school. I doubt the school would look too kindly on one of it's students being a dominatrix."
"Okay. I get it." She snaps, glaring daggers at you.
"From now on, you're going to do more stuff for me as well." You say, "Starting with this weekend. You wanted Dad to take you to that concert and he agreed, but I wanted to go fishing with him. You're going to tell him you've decided not to go to the concert, so we're free to go on a fishing trip." That seems to be a step too far. Elise looks apoplectic with rage.
"No way. I've been looking forward to that concert for months. I'm not cancelling. Go ahead and show the video to my mum, I don't care."
"Are you sure?" You reply teasingly, "Wouldn't that ruin your image of being Mummy's perfect little princess?" Elise lunges for your laptop, but you dodge her and snatch it up into your grasp.

You face each other, locked in a stalemate, each waiting for the other to break it.