I'm sorry to hear that your MIL has been left high and dry. Is your step FIL still working? Maybe your MIL could get an emergency hearing or something like it, to get immediate support from the bum.
There is probably some senior citizen advocacy group which you could contact, to see if there are funds available for your MIL to receive round-the-clock care in her own home. Otherwise, the alternative is to help her move to a nursing home.
Here is a link which will help you locate a few local organizations:
Experience is a wonderful thing.
It enables you to recognize a mistake when you make it again.