Think I've been molested

This is a somewhat recent development but something I've had in mind for years. The only problem is, I sort of always felt like it might have been my father given his history with sexual abuse. I never wanted to say anything because I wasn't certain and with him and my sister I just sort of felt like people wouldn't want to hear about it so I always kept quiet. Then I started to realize something about everything I've forgotten - they don't revolve around times when I was with my father. There's more to it than that, but this is just recently starting to surface. The person I'm talking about is still around and I'm honestly terrified of this person and I'm feeling kind of scared right now that they'll find out which is unusual for me anyway. I'm around this person and know they're not stable or rather weren't stable at the time this would happen, they act oddly different now. I keep quiet about it but it really would explain a lot. It really pisses me off because I'm around this person on a regular basis and we just talk - but I've always had to fake being happy around them but I have mixed feelings about this person. More than anything I'm terrified of this person. This person does things in private (or at least used to) but when this person is around people, they're a bit different. I remember trying to warn people about what was going on before but when they investigated, they just lied about everything. I don't know what to do or whether I should act. I just want to know for sure first and didn't know what to do.