Dad is thinking the same thing as you are, and you both barely manage to cling to the laces of one of Jenna's flats as she arrives and picks them up.
You are jolted upwards as if on an amusement park ride, except that in this case you must hold on desperately to a string of fabric in the hope of not falling or being thrown off.
The world seems to pass by in a blur as you spin with the shoelace. You close your eyes, feeling your stomach churn, and you can feel your sister dashing back up the stairs.
After several seconds which seem longer than they actually were, Jenna has arrived in her room, and she tosses her shoes. This last fling sends both you and your dad flying across the room. You both cry out as you soar, tumbling through the air, until you land and bounce upon something soft. Lying face-down, you are only just regaining your composure when you are sent bouncing up again. This time, when you fall back down against the soft fabric beneath you, you can see that you and your dad are on Jenna's bed, which she has also laid down upon. However, she has stripped out of her pants, which now lay in a bundle near the closed door, and her panties are nowhere to be seen.
You unknowingly stare as you see Jenna's pussy for the first time in your life. She is completely shaved, and her legs are spread and bent, with her bare feet flat on the blankets. She caresses her clit and your mouth slightly falls open. A tremor shakes through one of her legs and she stretches out, moaning quietly.
Then, as you remain unaware of yourself, you see her insert her fingers into her vagina. She murmurs, trying to remain quiet while also feeling very aroused. A strange sensation passes over you as well as you see your sister pleasuring herself. You feel the blood pulsing into your dick, but you are very conflicted. This is your sister, but seeing her getting off is also getting you going.
You finally return to your senses when your dad grabs your shoulder.
"Come on, son! Now's definitely not the time to get her attention!" he says to you.
However, as you both turn away from Jenna, she stretches out her leg and unknowingly lays it down upon you and your dad. Trapped beneath her calf, you both are unable to get free. She then reaches into the end table beside her bed and pulls out a skin-colored dildo. When she removes her leg from upon you, she sits on her heels and inserts the dildo into her pussy. You and Dad start to run again, but Jenna's sudden ferocity causes the bed to shake, and you are both knocked over. Even worse, every bounce of the bed sends you and Dad rolling back towards Jenna, whose body sinks into the mattress.
The moans become more frequent and you both continue to struggle not to fall underneath of Jenna.