Only read the OP so far but I can't tell you enough how brave you are in doing this and starting to deal with what you went through.
I know a bit about you as you have posted on my threads before so I hope I can help you in some way.
My sil told me and my husband last september what she went through from age 4-17 with her dad - truly horrific. 2 days later she found out for the first time after 30 years that he had done the same to her little sister. Massive trauma and upset in the family. Few months later went to the police, long old process of interviews and video statements etc. He denied everything and said they are making it all up.
He is then charged on 48 counts and in court he pleads not guilty to all 28 (they condense them) charges of assault, rape and buggery). On bail now until the crown court hearing later in the year.
Both have found it difficult, I won't lie and say it isn't a tough thing to tell. They had many years of living a lie, seeing him regularly, pretending he was a great dad etc. Its a big step to take but they have found it liberating and as though they have broken free. He made them so scared when they were children that they took all those years to tell! Well, I kind of forced the issue as I had some suspicion and said she had a duty towards my children not to have them at risk.
I will forever be indebted to her for telling us, god knows what I would have done if she hadn't of told us and he did it to our girls.
Your step father is a sick evil freak and he could be doing it to someone else so for that reason you should report him to the police. they have been marvellous and made a very painful process bearable.
Mum is a different issue. They plucked up the courage to tell her and she has rejected the oldest mainly and hasn't even mentioned it ever again to either of them or her son. I dont' think she believes them but on the other hand I understand what a big thing it is for her to accept and comes to terms with. That is still an issue.
If you are going to go to the police then there will be fallout. It may damage other relationships you have, like the one you have with your mum. Your feelings should come first and foremost but sadly that may not always be the case. You need to be prepared for that, and that you may lose people along the way. BUT, remember you are doing it for the right reasons - to stop him from doing it to anyone else ever again.
You don't have to rush into making decisions, take things slowly and keep asking for support.
I hope this helps, I am being completely honest as I would never want to fool anyone into thinking it will be an easy ride. I am of course hoping to have a positive thread on here about a conviction in the future.
Do you know what felt really good - knowing he was at home yesterday with no fathers day cards or presents as his children all hate the vile creature he really is.