Fear of incest was a major concern in the discussion I referred to. The ones who had shared rooms, however, had never heard of it happening. I searched the literature that I could find on the subject, and it is clear that the only cases where it might happen is when a brother and sister didn't start sharing a room until near or after puberty. In particular, if they had started sharing a room well below the age of five, you could expect zero cases of incest.
It turns out, however, that typical American child rearing practices maximize the chance of incest. A lot of the research didn't ask the right questions, but there is enough to piece together how humans avoid incest, and the Israeli kibbutzim did it perfectly. In studies of them, there were zero marriages between children raised in the same kibbutz. The children, aged up to five, had an hour or two of unsupervised time between waking up and being taken to the communal showers and then getting dressed. They were naked at that time, and sexual play was common.
The research isn't adequate to know whether it was the nudity, the sexual play, or both that did the job, but after they reached puberty they had no sexual interest in each other. They all considered everyone in the kibbutz to be a sibling.
With sexual repression and a bedroom for every child, incest is becoming a problem here, primarily because of the very high rate of birth defects--the chances of at least a mildly harmful defect is over 50%.
But, as I said, as long as they started sharing young, that won't happen.
To answer people like michaela, whose parents didn't destroy their curiosity, althouth I have earned my living in the compuer business, I took a lot of sociology and psychology in college, and got my start in the computer business working for researchers in those fields. So subjects like this came up in my college classes, and they weren't afraid of talking about such things in those days. I understand that today it isn't good for your career to get into things that aren't politically correct.