I think that you are asking my advice on whether or not you should testify, and based on your reaction I think yes. I think that you need to clean the slate, and while you certainly should do this to help the other girl and potentially others, I think that you more so have to do it for yourself.
The reactions you are having seem to be indicative of your wanting to be more honest in an attempt to purge yourself of this. That's totally my assumption based on the little information you gave me. But you seem to be saying ‘I want to help, but he's my step father’. I think you have to stick to the former, especially because you could also say ‘He did that to me and he's my step father’. With strangers we feel vulnerable, but not manipulated into having to help them save face.
Good luck, and let me know if I didn't accurately interpret your request.