Very little. The offence is defamation these days, slander (spoken) and defamation (written) have both been replaced with the one offence called defamation.
If she honestly believed that you had offended in this way, she is entitled to make a complaint to the police. Usually the police will investigate and if they find substance in her statement they will warn you or prosecute you. If they do not they'll do nothing.
What did the police do when they visited you?
If this person is continuing spreading damaging but untrue gossip about you, and you want it to stop, perhaps you should engage a solicitor to write to them and threaten prosecution if they continue. Are they still doing this, if not often the gossip will stop, however if not perhaps you should visit a solicitor to write to this person threatening that they will prosecute if the untrue statements continue.
Be aware that to take action through the courts may cost you a lot of money.
Unfortunatly that's correct, if it is her "honest opinion or believe" that you did such thing (or at least she claims such to be her honest believe and opinion), it could be very hard. Not saying that it will be impossible. They will also look at the actual damage and harm caused. Is there a well eatabished reputation which got destroyed due to her action? Did you suffer a financial loss due to her action? And so on... Now, its probably not worth to take legal action. You might have a small chance but you probably won't have the outcome you would wish for (financial compensation? Her paying in another way for what she did). The Legal costs and stress aren't probably worth it. Now, do you really want to sue your step children's mother? By the sounds of it, she will probably tell her kids that she has to go to jail cause of you.
You have my symphatys as I am a step mother as well. I am lucky my husbands ex wife isn't crazy and the only issues we have are access and financials (well they go par in parcial)... A personal recommendation I could make is, telling her that you are considering legal action against her. If the kids say something you can always say it isn't true. It may prevent her from making these accusations against you.