I too am new here and don't know how it works. While I find it difficult to know who is a perptrator and who is experimenting normally. For my self if the sibling is pubescent and the recipient of the sexual behavior is not, that is probably unfair, exploitive, and not an acceptable experiment. If the are both prepubescent and within two years of each other, I would probably let is go as a natural experiment. The same is true if they are both pubescent. Unless there is more coercion thjan just being the older sibling.
I am 63 years old. I spent about 10 years in cognitive behavioral therapy. I got good techniques for coping but no help in healing. I have spent about 6 years in psychotherapy and have accepted that I many of my defenses were psychotic delusions and that my lack of trust and my paranoia are between my ears and not from a totally hostile world. I was first molested when I was 10 and last molested when I was 17. My mother had me undress until I was naked to her photographer friend could take pictures. I was orally and anally sodomized by my mother's friend who was supposed to "take care of me." When I was 13 and 14. When I told my parents about this my mother asked if I enjoyed being penetrated. When I replied no, it hurt, she said then you are O K.
It was a physically abusive house, as well as sexually abusive. My blood poisoning from my hand up to my elbow is probably an indicator that I was also neglected. Generally, I have tried to ignore and discount what happened. "I was never severely abused... they never broke my bones." Today I know that is not a standard for measurement.
What I am struggling with is how to grieve my loss. Which requires that I acknowledge that I did not have safety. However, how do you have a loss that you never had to begin with. If I could believe that I had a right to safety as a child, I think accepting it as a loss would be easier. But I don't know how.
I hope this is an acceptable introduction of myself. It also is a little responsive to the previous posting in the struggle with denial that it really screwed me up.