Your sister starts to grow a dick. It isn't clear at first, she just seems uncomfortable and you kind of laugh about how much she's grabbing herself. But soon after, she comes back and shows off the small bulge. It makes her feel really good actually, and she pokes that even her small bulge is probably bigger than yours. She can't help herself really, she strokes it until she has this shaking orgasm in her panties.

She wanders off with a smile, like maybe there's more to come... and it's true. Her cumming just brought on MORE growth. Her cock is MUCH bigger, which she shows with that panty bulge. This time though, she doesn't just show you from your side, she decides to jump on the couch and hover over you. Tower over you with that massive bulge.

And she's stroking again, telling you to do the same. Go on then.