Looking for a brother sister story

[Johnny_Smith21]

I have spent months looking for this specific story I read awhile ago. The details are a little hazy, but what I remember for sure is it’s brother POV story with his sister(possibly step I don’t remember that part) and he shows up to some kind of party. The sister has a friend there as well. There’s a sheet hanging up and people are paying(?) to see people fuck behind the curtain. It’s an anonymous thing. The brother and sister end up behind the curtain and she tells him to dry hump her. He ends up cumming. Then the friend takes him upstairs and fucks him. Most of this is accurate but I’ll look into any stories sent that could be close just to see. Any help is appreciated.