It is commoner than one would like to think, sadly.
A brother makes it really nasty as there isn't any escape, and he could technically have ''babysat'' for you.
I was abused by a plumber that came to parent's house twice- I was 9yrs old, and parents had left me alone with him, asking that I ''be nice to mr Hull''
I had no idea what was happening, and it was awful.
I didn't ''tell'' as Mr Hull threatened bad things happening.
Parents when I eventually told them {in small detail, in my twenties just testing water} were horrified and tried to dismiss it.
Parents can react very strangely.
Guilt??
Who knows.
But please try to get proper counselling.{Specifically around child abuse as a victim}
I didn't get counselling and regret it.
It's normal to be thinking of it many years later, friends say the same.