A friend of mine passed along the link to this Reddit thread, started by a man who had a long-standing sexual relationship with his sister that began in their early tens. Of course, there's no way to confirm the veracity of the poster's claim, but if we're to take him at face value, what he has to say is pretty fascinating. Here's the original post:
I'm a 26 year old man who up until recently had an ongoing sexual relationship with my sister. Here's some general information:
We were both consenting. She was 13 when it began, I was 12. The relationship was ongoing up until last year when she met her fiance. As far as I know, neither of us were abused in anyway when we were young children. Aside from the relationship, we're both perfectly "normal" functioning members of society.
Ask me almost anything. Please refrain from requesting any kind of identifying information, as I'll just ignore the question.
As expected, many of the comments are very juvenile, but some of questions and responses are super interesting. Read it here (NSFW language!): link.