"Camping" by Losgud (lushgod@hotnomail.com).
http://www.dejanews.com/getdoc.xp?AN=349939576

The narrator is a writer or editor for an architecture magazine. He has got himself trapped into going on a camping trip with his younger adult sister. The ambiance is very nice. I found the relationship between the siblings before the sex to be a bit irritating, but it was also realistic. The camping trip is realistic - complete with rain and trail mix and all kinds of underwear. Eventually the narrator develops a new kind of relationship with his sister. The sex is quite nice!