I know how you feel. It was only 2 weeks ago that I found out that my half brother had also been sexually abusng his half sister on his mother's side. I have really struggled with that knowledge. It made me feel like vomitting. It somehow made him more of a monster to me. A REAL pedophile if you like. I had thought it was just me and my brother up until then. I am just glad she told and he was convicted for what he did to her. I wish I could talk to her but I don't even remember her name (I have only met her a couple of times in passing)
Like you I wonder how many more there are that he hasn't told me about. I'm not sure I want to know either.......
More than meets the eye!