My older brother assaulted me multiple times as a kid

When I was a little kid, maybe 9-10, my brother started to assault me multiple times sexually. We are both adopted. He would call it a “massage,” and when he did it, he put a blanket over my backside and told me not to look back. He did this for at least a year, or honestly, I can’t remember much of it because I’ve convinced myself it never happened. He did it at our house, grandparents, on vacation. He eventually stopped, and a kid, I never knew what happened. Then, when I got older, I started to realize what happened. I told my friend, and somehow it returned to him. He confronted me one night saying he was so sorry and that he was addicted to p*rn at the time and didn’t know what to do. Our parents were going through a nasty divorce at the time, and my mom ended up dying before the divorce was finalized. He begged me not to tell our dad because he respected him so much and didn’t want to loose his family. I don’t know what to do. I’m an adult now, and this has been on my mind since I was a kid. I’ve never told an adult before, but don’t know if I should tell my dad. I hate my brother for doing that to me, but I have this underlying guilt for him. I don’t want him to lose our dad either. But what he did to me was not right. I don’t know what to do, but my best friend says I must tell my dad it’s the easiest thing in the world. I’m thinking of telling my therapist first. It’s not like I forgive him, but I’m not sure.