I haven't read the whole thread but I wanted to put my own experience on in case it is helpful to you.
When I was 15, my mother's husband seduced me. He made sexual advances, and I did not reject them. I in fact felt very attracted to him, and felt very guilty as a result. I eventually told him I did not want a sexual relationship with him, but he continued to make advances.
I ended up leaving my mum's house and going off to live on my own at 16, as the situation was intolerable. However, I still felt a lot of affection for him, and continued to have a close relationship of sorts with him. I also felt very sorry for him. A couple of years later I moved abroad and pretty much stopped seeing him at all.
I told very, very few people about this until 8 years later, when my younger sister turned 15 and made allegations that he was molesting her. My mother had trouble believing it (she had been with him since my sister was 6 years old) and I felt I had to tell my story in order to support her.
I felt terrible about saying anything, and would not have done it if it hadn't been because I felt I would have been betraying my younger sister if I didn't. My mum left him then, and I felt very sorry for both of them that it was all over. I also felt a significant measure of guilt for what I perceived to be my part in all of this (of course looking back on it I do not think that I was responsible, what with my being 15 and him being in his 40s, but that is how I felt at the time).
I am not saying this is anything like the situation in your household, but maybe hearing other experiences is useful. By the way, I always used to balk when people talked about him abusing me, as I did not reject all of his advances.