So angry with immoral nasty dirty EX SIL

[Plentyapollo]

Ex (9 years ago) SIL encourages her 20 year old son to be physically and verbally spiteful to his step brother with autism & my son without. Calls them retards is purposefully hurting them by 'accident'.
She this week has followed my 81 year old MIL around the local town begging her for money and not paid it back... this is the third time this has happened in three weeks!!!
She still owes us £510, she has been evicted from her rented house after the landlord whom she was having sex with for years has now caught her shagging both of his married brothers... the best of this is that the landlord and his brothers have a sister that is married to her brother... I couldn't make this up.
Her son has stolen from my house my dh feels sorry for him as he hasnt had the most stable of upbringings!!! She left My BIL when son was 5 to run off with landlord thinking he would leave his wife for her.
After my 13 ds complained that they called him a spoilt little bastard and he violently twisted his fingers causing bruising last visit... I have banned them from our home.
Want to do something rash about her taking advantage of my lovely MILs vulnerability...I'm so angry

It's obvious that you are very angry about all this, and from what I can figure out I don't blame you. But other than protecting your nearest and dearest and keeping an eye out on vulnerable loved ones (like MiL), there's not much you can do about the way she lives her life. But again, this is your husband's mother she was bothering, right? So what is your husband and/or his brother doing to protect their mother? If you all feel exSiL has crossed a legal line into elder abuse or issuance of threats/intimidation, see a solicitor or the police about a restraining order.

At one point my own mother felt intimidated by cold callers or high pressure salespeople and was agreeing to things just to get away from them. We sat down with her and told her we loved her, and that she was to simply say "My son/daughter handle all my finances now. You'll have to talk to them, they write all my cheques" and to give them our phone numbers. It wasn't true (at that point) but it did stop them harassing her once they thought she couldn't give them money. And funnily enough those salespeople never called us. Maybe MiL could so something similar?

As far as 'exposing her publicly' what do you want to do, take out a full page ad in the Times? I mean, chances are anyone who comes within her orbit figures out 'who she is' pretty soon. Obvs if someone asks you if she's trustworthy, good as a friend etc etc, you tell them the truth. But that's about all you can do. To make a big stink will only give her the chance to portray herself as a 'victim' of your family's meanness to her.