The door opens a crack

A soft knock at the door freezes both you and your sister. "Gwen?" came your father's voice from behind the door. Just as he always did, your father began slowly opening the door immediately following the knock. You and Gwen jump to cover yourselves before anything was seen. Four things happen in a very short period of time:

1) As your sister involuntarily jumped, her dick -- whose head was lodged at your entrance -- slides completely inside in one stroke.

2) You moan out, eyes bugging out at the unexpected intrusion. Your sister groans at the unexpected pleasure, her eyes also wide. The feeling of fullness immediately blanks your mind of everything except for the fire of lust burning in your crotch.

3) Gwen still had enough of her senses to pull the covers over both of you and yell, "Dad! Just a minute!"

4) Your father sees through the crack that the two of you have your crotches pressed together. He assumes the two of you are rubbing your pussies against each other. He hears your sounds of pleasure and Gwen's shout; he quickly closes the door.

"Er..." he starts, somewhat awkwardly. "Breakfast is ready whenever you're... uh... done." He knew Gwen had never had any boyfriends, but he never suspected her to be a lesbian. No one could get pregnant from it, so he may as well let it go on. Who knows, maybe they're bi and open to something with one more person...

Back inside the room, you remain straddled over your sister, her dick pressed balls deep inside of you. Both of your hearts are beating faster than they ever have.