i believe the relationship dynamic have a lot to do with it, and also the perspectives of those involved.
in my case, my older brother [ 26 months] was the eldest, and was a god-like figure. he was preferred one, and my mother even said, 'everyone knows the first one's special', so that there says a lot [ if you know what i mean....... the jury is still out on that one...]. anyway, he was often put in charge of the rest of us when both parents were away at work.
over the course of time it was revealed that he was having sex with 4 of us sibs, two of which i walked in on. the 4th one kept mum about it until about 10 years ago, then it came out.
at any rate, i found out my older brother was having sex with the brother 26 months younger than me, and then he and i started having sex, but i can't remember who started it, or what the circumstances were that led to it, or how often it happened.
as it turns out i could have been considered a perp, because my younger brother was younger than me, but from my perspective, i did not see myself as any more powerful, but rather as an equal. however, from his perspective, i was a 'bigger' brother. so in that sense, i was a both a victim and a perp. incest is a very troubling and confusing thing. everyone has a different perspective of where they and others stand in the pecking order. for me, in retrospect, i did not see sex with either brother as being about power, but rather about getting intimacy needs met in a family system where intimacy was always negative. for my older brother, i can't begin to imagine where he [ a 15 y/o] was coming from, or what was in his mind as he was initiating 4 of his sibs, the youngest a 4/5 year old little girl.
the whole thing was, and still is so confusing, and even tho the older brother died at age 23, the rest of us are still alive, most unhappy and medicating with pills and other stuff.
everyone is satisfied with the status quo, and as long as no one talks about it, as long as no one rocks the boat, everything is 'ok'. for them, that is. me on the other hand, i have a very hard time dealing with the denial and the hardest thing about having relationship with this group of people is the fact that they just want to ignore the damage, yet pretend that everything is ok and that makes me an outsider; a stranger in a roomful of people whom with i share the same psychic history.
it's a nightmare from which i'll never wake.
all the best,