I was molested as a child, would it still be right to tell my parents?

Hi My name is Sarah,

When I was a little girl, about 4 my mother moved in with her boyfriend "Brian". Things were alright at first but as time passed on he began to abuse me physically and mentally when my mother was at work. He would mostly beat on my older brother who stood up for himself. My mom has always thought that my brother got the worse of the abuse. But I have a secret that I have kept for a very long time.
Around the time when I was 5, when my mom wasn’t home, Brian would make me go into his bed. He would fondle me and make me touch him as he touched me. As I grew older it got worse soon he was "Kissing" my private area and making me do the same to him. This in time leaded to sex, I was very young and very confused. I didn't understand what was happening, and when he told me not to tell "mommy" I didn't. This lasted until I was about 7. When we finally moved away from him and I grew a little I learned how wrong that was. I was embarrassed about it and too ashamed to tell anyone.
I tired forgetting about it for a long time; I didn’t tell anyone until I was 13, but then the guy I told didn't seem to care. He made it seem as though I shouldn’t complain that I was molested as I child because it happens to many girls. No parents know about this.
The man who molested me is my half sisters father.
I want to tell my mother, but I'm afraid. I also have a horrible relationship with my half sister because of this. I have not had any therapy for this.

Should I still tell my mother? Will it do any help ?