What an unenviable situation you are in. I have never been in this situation myself. However, I do have some thoughts on the matter which I would like to share with you.
It seems to me that people like your stepfather are never too old to be a risk. I think any man who will molest a child, even once, is fundamentally flawed.
I do understand how you feel about your mother. However I think she has had a lot of years without worry, while you and your sister have had to deal with things that you should not have had to deal with and that in fact, she should have been helping you to deal with. Also, about our stepfathers own family...are you certain that he did not also abuse them?
Jen, ultimately, despite all the concerns you may have and the 'what if' questions, my thoughts are that you and your sister have to do what is right for you two. You are the most important people in this situation and you must do what will set your minds at rest and give you closure. I don't really believe that anyone else is in the position to tell you what you should do in this situation. This is about your life. However, I know that I would like to offer my moral support (and any other support I can) to you no matter what you decide to do. My very best wishes go to you and your sister Jenty. Please be in touch if you need to be.