After waking up in the middle of the night feeling thirsty, you make your way downstairs to get a drink of water.
Reaching the bottom of the steps, you hear a soft, strange cry. A tender female voice drifts in from the next room, barely audible above the low hum of the AC. You peer around the corner and see your sister Riko, wearing only an oversized t-shirt and lying face-first on the couch with her butt up in the air. The t-shirt is bunched around her upper-torso, leaving her lower half completely exposed.
Both of her arms are reaching down between her legs, with her right hand moving a sharpie in and out of her dripping pussy, and her other desperately rubbing the top of her pussy.
There's another moan, more distinct now that you're closer but still muffled as you see she's moaning into the couch cushions to dampen her voice. Most of her face is hidden, but you can see her eyes are glazed over in a sexual frenzy.
SNAP
The floor settles slightly under your foot as you shift position, leaning around the corner to look more closely.
She jerks her head in your direction. Seeing you there, her eyes widen and she freezes, her slick grip slipping from the marker enough that it pushes out of her onto the couch, a trail of fluid dripping down after it.