[REQUEST] Obscure old book

I’d like to find this book I read ages ago, but I don’t remember the title. What I do remember is that it was about a family that goes on a camping trip, and basically ends up all having sex with each other. I remember there was one scene where the brother gives himself a handjob with his sleeping sister’s hand. I also recall at one point a Japanese guy showed up with his wife and they joined in. I know it’s a longshot, but does anyone know this book?