when children abuse

I have been once again confronted with the spectre of sibling sexual abuse. typically an older brother sexually abusing a younger sister. I interviewed a woman who was sexually abused for many years by her two year older brother. he demanded she do it, she did it. He invited his best mate to join in. the abuse occurred about two to three times per week for about five years. it occurred when mum and dad were at work in the afternoon after school. it began when she was eleven and he was about thirteen-fourteen. it was, from the start, full intercourse. she disclosed to her mother. she was told she was a liar. the abuse continued until she left home. needless to say she has been severely damaged by the experience.

The issue raises for me so many questions. Does it happen in all families? is it a consequence of opportunity, will all boys do it if given the opportunity? does it only not happen because of something about the girl? does it require a particular dysfunction?

families seem to need to be somehow disconnected, but i fear it is not just occurring at the very dysfunctional end of the spectrum. i worry that this might happen in nice middle class families if opportunity is given.

i wonder also when does sexual play constitute abuse? dose she have to say "no" for it to be abusive? if she pulls down her pants, is never threatened or hurt is it play not abuse? is it ever play or is someone always in charge? is a child aware enough to know the difference? that is the offender as well as the victim? is the family always the system at fault?

so many questions, a feel a phd coming on!