I can only give my reasons why I never told anyone in my family about it because every family is different.
My reasons for never saying anything have changed over the years. When I was a child I never said anything as I knew it was a secret and didn’t have a close enough relationship to my parents to ask them about it. As a teenager, after it had stopped, although I knew then that it had been wrong, I kept quiet about it as my home was a very happy one and I thought that if I said anything, my parents would split up and I didn’t want to have to tell them any details about what had happened. And now I have a family of my own, I still would never tell them. My reasons now, are that it wouldn’t make me feel any better, and would devastate my parents. It’s easier to keep the secret from them now I’m older, but as a teenager, carrying that burden was very hard.