Yes, an adult doing ANYTHING sexual with a child under the age of 18 is abuse! (Save those loopholes where someone 18 can date someone within so many years of their age in certain jurisdictions and have it be perfectly legal, ie an 18 year old dating a 17 year old.)
Children are brought up to think that whatever environment they are in, that environment is perfectly normal. Many people grow up believing that everyone is hit or yelled at or whatever..... Its not until later on that a realization is made that the childhood was indeed abusive.
Here's some perspective for you. Until I was 28 I thought that my childhood abuse was no big deal. (Molestation.....err, I hate that word as it sounds like I was merely touched inappropriately. It was indeed rape.) I knew what happened to me (no lost memories) and thought that it didn't affect me. People have gone through a lot worse than I have, and they have also believed that the abuse didn't affect them. Delayed onset PTSD is quite normal in the world of PTSD. Many of us go for years or even decades thinking that what happened to us was no big deal and had no effect whatsoever.....until it all comes crashing down and we realize that what happened really did effect us in ways we never even realized.
Your mom is in denial. Many parents are in denial. Some cannot fathom the fact that something horrible would happen to their child. Other parents simply do not care but masquerade around like perfect parents, thus fooling everyone.....but when it comes right down to it, they wouldn't do anything to truly protect their children.
Your mother failed to protect you. I know its hard to accept that, but if she allowed this behavior, she's just as guilty. If it happened without her knowledge, that's a different story and she may be in denial after the fact.
The truth is that as a child, we are forming our sense of safety in the world. You had unwanted touch forced upon you......this sort of thing can indeed cause long term safety issues. I was violated as a 3/4 year old, and I'm pretty sure I'll have issues with feeling safe for life (even though I've come a long way in my healing already). Its one of those things you can't undo, can't fully fix as it was a crucial point in development that I completely missed out on.
ETA
In terms of not being able to remember.....Well, we can store trauma inside of us from any age. That means infants who are abused can have issues later in life. Birth trauma is a very real thing for the child (not just the mother giving birth). And I don't doubt that trauma to a mother can indeed affect a child in the womb.
What I'm trying to say is that it doesn't matter that you can't remember your trauma.
