Looking for story: Incest/Swinger

I remember reading a story a few years ago that was about a sister going to visit her brother and sister-in-law and found a book in their room full of pictures of them fucking other people.
The two lived in a swinger community, but the girl didn't know that yet, and finally I remember that the sister was able to lick her own pussy(auto-cunnilingus).

If anybody knows that name of this story, could you please post the name or link?

Anybody at all know the story?

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That would be it, thank you very much. (I can't believe I missed that, it's by one of my favorite authors even!)