Looking for a story about a guy who follows his sister out at night.

[Niveous]

The main details that I remember is it's the sister and two other women plus a guy. The sister and one of the other women pleasure themselves and the guy as the third woman sinks in mud. All while the brother is watching, mortified, from the bushes. Don't remember who the author is and I could have sworn I had it saved somewhere, but I guess not.