You don't have to do anything. You were the victim of something horrific and what happens or doesn't happen to the perp was not then, is not now and never will be your responsibility. The only responsibility you have is to protect yourself and your kids, which you've done. Whether you re-report or not should be based on what you need to do, for yourself and your family. You're a victim of an specially nasty crime, and the police let you down once. In all honesty I would be amazed if they let you down that badly now, but the point is you don't owe anyone anything on this front. So do what feels right to you.
Secondly, if she did tell SS a pile of steaming bullshit, you explained the back story, and they learned you reported this at 15, then SS' main interest would be in knowing you immediately protected the kids from him. The culture has changed a lot and their standard of evidence is massively lower than criminal, anyway - potential risk to the kids from a woman who ignored her own child telling her she had been abused exposing them to the abuser would be their top worry. But she won't go near SS - as someone else has said, the last thing she wants is to open that can of worms. She just wanted to hurt you for not going along with her fantasies. And if she reported some lies to the police and you state she brought a man you reported years ago for sexually abusing you into your life and your children's lives without warning, let alone discussion, then all else she says is unlikely to be believed from then on out - the report will be on file, I imagine, so her spite would be dismissed.
Glad you pushed her out of the house, incidentally, though I'd be cautious in phrasing it that way if she did scream assault to the police - say she demanded to stay and you insisted she left but no violence was used - your DH was there to support your statement of events while all she has to support hers is spite and her own history of maternal failure. And I'm afraid I agree that you need to go no contact - she is incapable of putting a child's interests firsts, ever, so why expose them to her? She allowed a man to abuse her children. Nothing can ever justify or excuse that. If your mother is hard and toxic for you to handle as a grown woman, why expose your kids to it? Eventually she will turn on them for not meeting some perceived need, too.
I agree talking to your siblings might be helpful, depending on the relationship. I'd also say that he may have form you don't know about, and other reports against him. Who knows. But again: you don't owe it to anyone to report him after the way you were let down in the past, and only you can know what would be best. 
I'm so glad you married someone sane and lovely and have a decent and fulfilling family life now.
I hope you have a nice hot bath and a cuddle with your DH, and feel less stress tomorrow. 