His dairy.

Your eyes widened as you see that she was reading your dairy, a little black book the you mainly used to write your various fantasies in so you could read them again later. And unfortunately for you almost all of them were about Crystal.
"Crystal that's mine what are you doing!?"
Crystal smirks at you as you go to snatch it away from her then simply said "stop" in a hard voice you've never heard from her before, and obediently stopped.

Still smirking she lifted the book, waving it tauntingly in front of you, before suddenly standing up, simply saying "stay" before leaving the room.
Not even thinking of disobeying her you waited until she came back, carrying a black dress you recognize as something she wore when she was younger, to tiny for her taller and filled out frame now.

"Dougie I had no idea you felt that way about me, you've made me so happy." she said her expression still a smirk. "You know you made me so happy I'm even willing to give you a chance at me."
Your eyes widened, your early concerns forgotten, the chance at her? Whether she meant actual dating or just being railed by her as her person sex-toy you don't really care at that point, looking up at her, her beautiful face framed in long red hair you were ready to do just about anything she said.

"This will no longer fit on me, but your thin and smaller then me" she said showing off the black dress she was carrying. "Wear this for me, go change right now, and I'll take you on a nice romantic date, and if your a good boy and do everything I say, I'll take you back here, to my room, and ruin your ass." And with that she threw the dress at him, it'd probably not even reach half-way down your calf's if you wear it.

Do you refuse trying desperately to deny his sister, or go wear the dress? You could also go all the way, go wear the dress and take some of Sarah's high-heels and make up, maybe get some extra points towards being fucked.