Molested by my stepfather

Ladies I need to know why to do. When I was a child at about nine or 10 years old my stepfather had molested me just once. There was another time when I was 15 when he propositioned me to basically have sex. He came and got in my bed one morning and told me that my mother had told him to come wake me up. Basically he was crossing the line again by getting in my bed and seeing where things could have gone. I'm now 25 years old and I have put this out of my mind for so long because I was afraid of what would happen if I came out and told the truth. I moved out of my mothers house when I was 16 years old so I haven't really had to deal with it. They don't even live in the same town as me so I really have been able to ignore this pretty good my entire life. Well things have changed because I am now 26 weeks pregnant with a little girl and I just couldn't fathom allowing him to be in her life.  Here is where it gets really tricky, my mother is completely dependent on him. She has a lots of health issues  and could absolutely not to make it on her own. I feel like if I told her the truth that it would literally be turning her world upside down. How could she live with her husband knowing that he had been inappropriate with me before? Where would she go? What would she do? Right now I'm not talking to my mother because we have our own problems of our own but I feel like it would be totally unfair to keep my daughter from her for the rest of her life because of something that she did not do. I really just don't know what to do anymore I feel like I can't hurt my mother and turn her world upside down and possibly risk her having more health problems because of what I would tell her but at the same time I can't do this to myself either. I struggle with this every single day of my life and it is just killing me inside that I cannot heal because the truth has not come out. I'm asking for prayers, good vibes and advice if you ladies would be kind enough to give it to me