I told my AODA counselor and the therapist I was seeing at the time (who work at the same place). I actually was in a mental hospital for kids and teens at the time, because the PTSD and depression were getting so overwhelming that I was suicidal. I met my best friend there; we are "story twins" because the same thing happened to her as to me, though by different people and at different ages (this of course means we are closer than sisters could ever be--it's an amazing bond). Her family knew about her abuse, and she urged me to tell about mine. It was very hard even to tell them as much as I did, and all I told them was that I'd been abused (in every way possible) by my brother from ages 7 to 9. At first, they said they wouldn't tell my parents; the next time I saw them, a week later, they'd talked to the people in charge and said they were required to report it, at least to my parents (I was 15 at the time). I couldn't even be in the room when they told my parents. I'd been discharged from the hospital only days earlier but (of course) knowing that my parents would know this horrible, horrible secret of mine that I'd repressed for so long was too much to bear, and I became more suicidal than ever. I stayed at my aunt's that night, away from my brother, and was readmitted to the hospital the next day.

However, my parents took it better than I expected. They weren't mad; they were horrified that this had happened in their house, while they were away or sleeping. Though both of them have pretty serious emotional problems themselves, they've been mostly supportive (except that my mother occasionally tries to force me to give her the details, though I haven't even been able to tell my T or my story twin those things).

Like someone else said, you won't know their reactions until you actually tell them. Most likely, they won't react nearly as bad as you expect them to. It depends on your relationship with them.

(Sorry this was so long)