Notyetgran, this is something I tried to rationalise over the years. When I was little, I felt that I had a dirty secret and that I was as much to blame as my sister. Then, when I got older, I told myself that all kids play "doctors and nurses." Not that what happened was just on the "I'll show you mine and you show me yours" level. It was much more than that, and I found it incredibly difficult to put into words to my counsellor, so I'm not going to attempt it here. Enough to say it was intimate, intrusive and has undoubtedly affected my sexual experiences for the rest of my life. Anyway, in my teens, I wrote it off as "play" and buried it away.
It was not until I had my own daughters, who also have an age gap of several years between them, that I looked at them and realised that it would not be normal for the older one to do such things to the little one. I also realised that it wouldn't happen with my girls - they wouldn't even shower together unless they were both wearing pants! - and that big sisters didn't normally do these things to little ones. I saw that something was wrong but I didn't want to think about it, so I buried it away.
It isn't easy, though, to keep things buried. In later years, there were a number of occasions where my sister made remarks that seemed like sheer hypocrisy to me - where she would condemn someone, particularly a child, for molesting another child (she had a friend who worked with disturbed children, who used to tell her these things) and I wanted to scream, "But you did it too!" She also said that a friend's sister who accused him of abusing her when they were kids (oddly enough, when he was 10 and she was much younger)was a liar who had been "put up to it" by her husband and psychiatrist. (Never seemed to occur to her that it might have been the reason why she needed a psychiatrist in the first place.) She was vehement about it, although she had never met the woman, and I had a horrible feeling she would say something similar about me if I ever dared to open my mouth.
Would I ever have confided in anyone, if my sister and I were not estranged? I don't know - maybe not. But I did, and my family were appalled. They considered it abuse. I went a bit crazy for a while, and my daughters took me to my GP, who sent me to a charity dealing with women who had been sexually abused as children. The lady who listened to my story in the first place, and the counsellor who listened patiently every week for a year, were very kind. They were in no doubt that it was abuse, given the age gap between us, the seriousness of what was done, and the fact that it went on till my sister was in her teens (even after she had started dating the man she would later marry.) I don't know whether it is abuse when a 10 year old does these things to a 5 year old, or if it changes at some point between 10 and 15. I suspect that, if she had been a boy, it would be more readily identified as abuse. That is another thing that makes me feel weird about myself, the fact that it was incest between sisters instead of brother-sister, which seems to be much more documented and recognised. The counsellor pointed out that girls and women can be abusers too.
So...basically, it is a question that bothers me and makes me doubt myself sometimes. Then I look at my kids, and my grandkids, and think, "No...what happened wasn't normal."
Sorry, didn't mean to derail the thread.