You peek through the crack.

You look throuh the crack in the door and se your sister laying on her bed. She is wearing a nightie and has it hiked above her hips. Her hands are alternatively frigging away at her moist slit and squezing at her breasts. She moans softly as you watch her unnoticed... for now.

Her hands are sopping wet as copious amounts of fluid run from her dripping snatch. You begin to think how hot it would be if she were fertile right now. There is an almost imperceptible change in the way she is moving and attacking her clit. Her hands come away from her slit for a few moments and you see her begin to play with what looks like an egg white mucous. It sticks between her fingers as she flexes her hand and stares at it wonderingly.

You remember from Health Class that that sticky fluid almost certainly means that your sister is ovulating... and from her reaction it seems to have happened just now.