She's putting on one of your condoms

You're still out of your wits when you hear Gwen say, "That was way too sexy." You lay there in the afterglow of your orgasm, eyes closed lazily, soaking up the feeling. After almost a minute, you hear Gwen say something else, "How do these darn things work?"

You open your eyes slightly and look over at your sister. She's fiddling with something on the tip of her dick. You take a closer look and see that it's a condom. "Do you have it inside out?" you ask. She turns it around and tries again, the rubber ring easily sliding down her shaft this time. The tip is a bubble, filled completely with air like a pointy nipple sitting on the end of your sister's prick. A drop clings to the end: a glistening pearl of your sister's pre-cum.

You comprehend the fact that she wants to have sex as badly as you do.