Girlfriends stepsister catches you staring pt 2

Your girlfriend's stepsister Sierra found out you're a pervert who looks up girls skirts, and has been teasing and taunting you with that for weeks now. Giving you little glances here and there when your girlfriend isn't looking, Sierra has been driving you wild, yet the two of you are never left alone for more than a second. Tonight your girlfriend and Sierra are hanging out, but they haven't decided what to do yet, so Sierra starts making suggestions and giving you flashes up her skirt again. When she suggests flashing the bouncer at a bar so she can go in (she's still only 18), your heart skips for a while. Just when you think you can't take the tension anymore, your girlfriend leaves the room to take a quick shower. Sierra takes advantage of the situation and shoves her panties at you, making you promise to use them to jerk off later. You have an idea, but you're not sure if she'll like it.... what if Sierra tried on 3 pairs of your girlfriend's panties? Which pair would you want?